How normal is it to train with pain?

Hey, guys!

I’m a -120kg powerlifter and started training about ten years ago. This year I was able to perform a 312.5kg deadlift, a 210kg bench and a 260kg squat (in training). Sadly I managed to reaggravate an old back injury with new symptoms not only once but twice in one year. Every one of these reaggravations sets me back at the very least a month before I am able to hit close to PR weight again.
Since injuries/pain seem to be the main factor that holds me back from progressing, I am trying to figure out how to reduce the risk of injury and what role pain plays in training.

A simplified injury mechanism (doing too much, too quickly or too heavy) make sense for injury prevention. The problem that I am facing is that I have much more training sessions where I can feel some sort of pain that inhibits me in force output than sessions in which i feel that force production is limiting me. So I have a couple of questions for the readers:

  1. How much pain is reasonable for training?
  2. Do you reduce load/volume as soon as you see a pain pattern (e.g. repeated shoulder pain for many weeks with bench pressing)?
  3. Do you just ignore pain as long as it does not get worse?
  4. Is lifting without pain even a reasonable goal? (in this case I mean joint pain etc. not just the general feel of heavy training being hard)
  5. Do you ignore painful warmups as long as the heavy working sets feel okay? I have noticed over the years, that a proper mindset (and banging music) before starting a lift can supress pain during heavy sets. But since I get hurt so often, I wonder if I am going in the wrong direction with this strategy.

Sorry for the wall-of-text.
Best regards,
Lorenz