What are the considerations to make when altering logbook entries?
I couldn't find any specific regulations, but correcting errors or omissions in the time logged is fine. You should never change an actual endorsement, though.

To take a simple example, if your CFI wrote in 1.1 hours instead of 2.1 hours then I assume you wouldn't hesitate to correct it later because it's an obvious error of fact. In the same way, if your CFI didn't write in PIC time when he should have then there's no problem if you add it yourself. Provided of course that you're sure that you were indeed allowed to log PIC time for that particular flight.

If you want, you could add a comment or keep a record of changes on a separate sheet in case someone ever questions it. But for what it's worth, I have a number of corrections in my logbook and no one has ever asked about them. There's a high degree of trust involved in logging time because it's so difficult to verify the information independently.

But you shouldn't touch an endorsement. Only an instructor can make an endorsement and even if there's some mistake then you should ask the instructor to enter a new one rather than change it yourself. Endorsements get much more attention that time logged, and I think it would be tough to explain why you changed one yourself.

Finally, you mentioned your instructors' "thinking" or "ideas" about logging PIC time. I don't want to make assumptions about their intentions (and I'm not a CFI), but logging PIC time is defined by regulation, not by what individual instructors think. If you're still in contact with those CFIs, it would be a great topic to discuss with them. Perhaps they didn't deliberately omit the PIC time, they just forgot or overlooked it. Or maybe they did have a reason, but in that case they should at least be able to explain it to you with reference to the regulations.