For a 91 Civic LX Sedan, 157k miles, original springs and shock absorbers:

1.
If a car definitely has one "failed" suspension spring, do people here advise
replacing both this one and the one on the other side, even if the other side
appears to show no damage?

Or, given the age of this car and that for most of its life it has been driven
in the winter-salty Northern U.S., would many of you old hands just say: Splurge
and replace all four spring and shock assemblies?

Or just replace all four springs, if the shock assemblies appear fine?

Here's the situation:

The rear left (= driver's) side is lower than the right side by 3/4 to 1 inch. I
am almost certain that this is due to having a very large, overweight person
sitting in the car's back, left, passenger seat briefly about 2.75 years ago, as
the next day it looked tilted as I describe above. It has remained so ever
since.

The rubber casing at the top spring coils in the rear left is broken in a few
places. The metal beneath looks fine. On the other side, the rubber casing
appears to be intact.

The noises from the car don't seem particularly bad. There are some creaks when
I get in and out to drive it, but otherwise, nothing leaps out at me as
particularly suspect. (I am a slim person; 120 pounds or so.)

I hope to drive this car in comfort (physical and financial) for at least five
more years. I'd like to break 250k miles on it. I do my own repairs on it.
Recently I learned how to replace (and participated in same) a MacPherson strut,
including overhaul using a spring compressor.

2.
Advice on which spring compressor to seek is also welcome. I have the general
idea of these but am not sure Autozone's loaners will be sufficient.

3.
Lastly, can I get away with Non-OEM springs, shocks, and bushings, given that I
expect to keep this 42 mpg car only about five more years?

TIA