Use of modal verbs when writing in past tense

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:

[1] 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?

[2] He could go back.

[3] He could have gone back.

Could is the past simple of can, so I think that sentence [2] 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 [3], right?

If I’ve not made a mistake up to this point, then I’m confused. Because, if could have is correct in both [1] and [3], then how do I differentiate the two uses? To be clear:

• In sentence [1] I’m referring to the character’s past.
• In sentences [2] and [3] I’m referring to the character’s present.
• Every sentence must be in past tense since that’s how I’m narrating.

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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:

[4] It would just make leaving more painful.

[5] It would have just made leaving more painful.

Thank you very much in advance!