According to this statistics, the death rate per 100,000 is 880. Most of the top causes are diseases, many of them more common in old age. The highest non-disease related cause is accidents at just 35.
Whether or not your treatment also cures cancer (which is the 2nd top cause, contributing 205) makes a huge difference to the effects.
If accidents become the #1 cause of death, it is likely that many people become more risk-averse. This could dramatically change transportation (motor-vehicle accidents make up half of the accident-related deaths), leading to more public transport, faster adoption of autonomous cars, lower speed limits, much much higher penalties for speeding and reckless driving and so on.
We would also have to seriously confront the issue of suicide, which would overnight become the 2nd most common cause of death, going from 1.4% of all deaths to about 20%.
Psychologically, I would assume that at first people want to keep this new immortality and turn very risk-averse. But over time, a new effect would appear: Death from not-wanting-to-live-anymore. Some people will get bored of life after some hundred years. Some will have lost too many loved ones to accidents and suicides. Some just have nothing to live for anymore, especially if children become rare.
The main effect will be on the mind. Our mind was not designed to live forever. Many old people suffer from their memories. All the psychological stress and trauma you suffer in a life, imagine that accumulates. If you have PTSD, three different kinds of depression, five different kinds of acquired fears and a whole lot of bad memories that won't leave you alone, you will need a whole new way of coping with all of that.
There is also the effect of "new blood" lacking. Kuhn once said that a paradigm change in science takes one generation. The same is true of politics, culture, social topics. Ecology became important because a generation grew up on acid rain, Greenpeace and an awareness for animals going extinct that the previous generation simply didn't have.
In this I see the answer to your first two questions. Both politically and in business, you would have old leaders with tons of experience but potentially mental problems and most likely being stuck in "the old ways". The conflict between generations would become considerably stronger when the old generation never goes away.