I have to say, that I have quite a lot of troubles with my back.
Sit-ups are quite rough on my spine so I try to avoid them, or at least do them slowly with very correct form. Of course form is everything but I found other ways to build my abdominals that do not trouble my spine at all (L-sits, figure 8 with weight, plank...).
I do not know a lot about squat now but what helped me a lot was deadlift.
I started pretty conservative, about 30 kg. I did have pain that I have troubles to walk, sit or stand. After differend recomendations from different people (move, do not move, excercise, do not bend, no weight at all, strech, do not strech...) I decided that I'm not that old and start practise a lot.
Deadlift, militarty press, bench, chins, usual stuff.
Now about 1,5 year later I'm there trying to catch up with Crossfit and my PR in DL is 110 kg.
Back pains are mostly history.
But I realize that my receipe is not for everyone may be. Except the fact that I had to find my own way.
I believe that potentialy every excercise could be harmful without correct form, even deadlift and situps.