I'm trying to write a story in past tense (I'm using deep POV).
I have two characters who parted ways. One of them is regretting not spending more time with the other before leaving. Since he is thinking about the past, I wrote:
 Maybe he could have waited a few more days before leaving.
Later, he realizes that he has the possibility to go back to him. Which of the following should I use?
 He could go back.
 He could have gone back.
Could is the past simple of can, so I think that sentence  would be correct.
On the other hand, could is also used as a modal verb to talk about a possibility in the present, eg. "We could get some ice cream". If I wanted to employ this second use of could, but in the past, I should use sentence , right?
If I've not made a mistake up to this point, then I'm confused. Because, if could have is correct in both  and , then how do I differentiate the two uses? To be clear:
- In sentence  I'm referring to the character's past.
- In sentences  and  I'm referring to the character's present.
- Every sentence must be in past tense since that's how I'm narrating.
Finally, I would like to express the fact that going back would make leaving (later) more painful. As you can imagine, I'm not sure which of the following I should use:
 It would just make leaving more painful.
 It would have just made leaving more painful.
Thank you very much in advance!