Regarding lincoln, by all accounts if he were similarly wounded in 2022 he would have surely perished. At best, they might have put him on life support in an irreversible coma for some period of time. John Wilkes Booth got him good, sadly.
On the flip side, I think that Garfield would’ve lived with modern healthcare though. Maybe McKinley as well.
Garfield 100% would have lived. Even with 1881 healthcare he'd have had good odds with most doctors.
After he died, the autopsy found the bullet on the side of the body the doctor had refused any attempts to search. A protective casing of tissue had formed around it and the surrounding organs were in generally good condition. On the other side of Garfield's body, where the doctor had obsessively searched for the bullet, there were little tunnels all throughout his body and these tunnels were full of all manner of infection and disgusting things.