Anything on land larger than a cat will die
No ifs, ands, or buts. In the actual K-T, large animals across the board were wiped out with little fanfare. These include things like terrestrial pseudo-tortoises, cat-sized multituberculates and marsupials, and other members of groups that otherwise had low losses.
So your small generalists like raccoons, rabbits, and the Virginia opossum? They will die. They're just too large to survive in a post-apocalyptic neo-K-T hellscape.
This includes humans. Humans will not survive a K-T scale extinction, whatever you throw at them. It's thought that after the meteor impact you didn't have any new plant growth for as little as a few years to as much as a century after the impact. Most plants likely regrew from dormant seeds in the Earth, and even then it took a long time on a human timescale for "normal" vegetation to reassert itself (the immediate aftermath of the K-T is characterized by a "fern spike" where weedy plants like ferns were the dominant plants until trees and shrubs got established). Imagine years with no new plants. Humans barely survived one year without a decent growing season. Our species isn't going to survive off of seeds in the earth, hibernating squirrels, and whatever we can dredge out of rivers. Our megafaunal butts put too much demand on a post-crisis ecosystem for us to survive long-term.
The largest proportion of survivors will be from freshwater aquatic ecosystems
Freshwater ecosystem are what are known as "brown" food webs. They are food webs that are less dependent on the sun than either marine or terrestrial food webs, because if the sun is cut off they can survive off of dead or decaying organic matter and aren't intrinsically tied to photosynthesizing land plants or phytoplankton. Semi-aquatic land animals that are otherwise tied to freshwater food webs are more likely to survive. For example, in the K-T most crocodilians actually went extinct, the ones that survived were either those that lived in freshwater all their life (eusuchians) or returned to fresh water to breed (dyrosaurs). Even sebecids, which were mostly terrestrial in the Cenozoic, are suspected to have survived in the form of semi-aquatic freshwater representatives and re-evolved terrestriality, some of the earliest Cenozoic sebecids are semi-aquatic. Modern birds are suspected to largely be descended from waterbirds that could feed off of brown foodwebs until plants re-established themselves. Notably, modern birds (Neornithes), were a minority in Cretaceous ecosystems, whereas the dominant bird groups (Enantiornithes, Hesperornithes, Omnivoropterygidae, etc.) all died with no survivors.
This is why the fact that freshwater ecosystems are disproportionately affected by human industrialization and pollution is such a big deal, because we are basically killing our back-up drive for the biosphere. Expect higher than usual casualties compared to the K-T because rivers, lakes, and wetlands are some of the most degraded ecosystems in the present day, and therefore will be less robust to mass extinction events. I'd still rather be a shorebird than a land bird in this scenario, though.
Mass casualties even among the groups that survive
One thing that is often underappreciated in the K-T is it is not as if the dinosaurs and other megafauna were selectively removed from the environment and the mammals all got through with little casualties. Many of the groups we see as "victors" of the IRL K-T barely squeaked through the extinction. For example:
- Crocodilians suffered mass casualties in the K-T. Only generalized freshwater taxa survived. Casualties included the herbivorous Simosuchus, the mahajangachampsids, the terrestrial baurusuchid sebecosuchians, the small, generalized allodaposuchids, etc. Specialized freshwater taxa like the turtle-eating Brachychampsa also went extinct.
- Only one clade of birds (Neornithes) and possibly as few as five lineages (ancestor of modern palaeognaths, ancestors of modern ducks and geese, and Qinornis)
- Upwards of 75% of all mammal species died in the K-T, including many groups of multituberculates, placentals, and marsupials
- Insects won't care. Insects barely even blinked in the IRL K-T, the only mass extinction to ever significantly affect them was the P-T. You might get a big die off among insects that specialize in parasitizing or living symbiotically with particular plants and animals, like ichneumon wasps, bees, etc.
Generalized mammals will be more likely to survive than specialists
This is one that's kind of a no-brainer, but it bear repetition. Before the K-T extinction in North America marsupials and multituberculates were actually both more diverse in terms of species and ecological niches than placentals, which were mostly omnivorous generalists. Marsupials were the ones that included specialized fruit-eaters, most of the specialized carnivores, and even semi-aquatic and burrowing colony-dwelling forms. However, marsupials and multis were disproportionately affected by the extinction because of this (As well as the few specialized placental lineages like the zalambdalestids), and as a result it was the few generalized placental lineages that took over the place after the meteor hit.
Disproportionate extinction of arboreal animals.
Arboreal animals did really, really bad across the K-T boundary. Mostly because the forests they lived in all burned down.
Most survivors among mammals will be hibernators or can go into torpor
This is something that has actually been studied in some detail. One factor that has been suggested to have aided in mammal survival across the K-T, especially since so few of them are associated with brown food webs, is the fact that they could just hibernate through the extinction. So mammals that can hibernate or go into torpor to conserve energy will do better than those that cannot.
Similarly, mammals that burrow or dig their own burrows will have a disproportionate survival rate, since they have ready-made shelter for when the impact hits.
Complete collapse of ocean food webs
The real K-T showed a complete collapse of ocean food webs, with animals with a planktonic lifestyle or dependent on plankton as the base of their food web disproportionately affected. For example ammonites, which were the dominant free-swimming animals and had a planktonic larval stage, were wiped out with no exceptions. The dominant reef-builders (rudists and inoceramid clams) also all went extinct. With the exception of sharks and some marine crocodilians, all of the marine megafauna present in the late Cretaceous (ichthyodectids, carnivorous pachycormids like Protosphyraena, protostegid turtles, mosasaurs, plesiosaurs) was wiped out. Large filter-feeders (pachycormids) were completely wiped out, and notably none of the living large filter feeders (whales, basking sharks, megamouths, manta rays, etc.) seem to have origins that predate the K-T.
Wherever the meteor actually hits will be disproportionately impacted
You can see this in North America. Prior to the K-Pg North America had a diverse fauna and flora which included a large number of non-teleost actinopterygians, marsupials, multituberculates, gingko trees, and dawn redwoods. Then the meteor hit and just about sterilized the North American continent, made worse by the fact that the angle of impact would have showered North America with the worst of the debris. Most of the groups that had North America as their stronghold of diversity pre K-T are reduced to relict groups today largely because of that impact, whereas North American ended up being mostly recolonized by groups that had their centers of diversity in Asia (like placental mammals).
Winners
- Shorebirds (esp. seagulls
- Crocodilians (it's time to reconquer Earth...again)
- Rodents
- Small ground squirrels like chipmunks are of particular note here, as they are 1) hibernators, 2) burrowers, and 3) omnivorous
- Shrews
- Smaller opossums like Marmosa
- Native marsupials and rodents reclaim Australia from the foxes, cats, rabbits, and other Holarctic invaders
- Sharks (they won't win, but they won't suffer huge losses either. Just keep trucking on.)
- Grass (will do better than most plants because they are highly resilient and wind-pollinated)
- River turtles
- Amphibians (surprisingly enough despite predictions of acid rain there are little to no extinctions among amphibians across the K-T)
- Freshwater fish (no more humans to pollute their water, plus an entire ocean that's just begging to be re-invaded)
Losers
- Ungulates
- Xenarthrans (armadillos have a better chance than most, but they're still too large)
- Primates (no forests, mostly frugivorous, large)
- Carnivorans (the smallest weasels or civets might survive, but it's unlikely)
- Marine mammals
- Tortoises
- Coral reefs (will probably go completely extinct and be refounded by some other reef-building organism in the aftermath)
EDIT: With regards to your moon question, it just won't work. The moon right now isn't habitable, it has no atmosphere, isn't large enough to maintain it's own magnetic field, and unlike Titan or Europa isn't in orbit around a gas giant where tidal flexing can provide those things for it. Adding enough mass to make the moon habitable will make the moon stop being the moon, it's gravity would be so great it would either end up impacting the Earth (which will probably end all life on it by turning the entire surface into molten slag, akin to the Thea impact), or it will break free of Earth's gravity well and establish its own orbit (this seems less likely based on my understanding of physics, but I'm not a physicist or astronomer). Either way losing the moon means Earth loses its habitability, as it no longer has a large orbital body to stabilize its orbit.
