1. I understand "It's only comfort, calling late" to mean "It's only comfort that is calling late" (as in "it's coming too late", sort of some half-assed attempt at making amends).

2. "Come undone" doesn't mean "undo" but "let loose" (in Deutsch etwa "die Zügel schießen lassen").

3. "Pucker up" can also mean "Show your true feelings" (or "be honest already") and I think that applies here.
German colloquial expressions like "Jetzt mal Butter bei die Fische" come to mind.

4. "Something borrowed, something blue" refers to a marriage tradition, so it could be translated literally ("etwas Blaues"), but of course it also has a figurative meaning ("the only things I own are a borrowed love and my sadness").

As for the interpretation of the title, I think it means "no matter what I do, no matter what you do, nothing ever changes" (or "no matter how I could be, no matter how you could be"), i.e. their relation is doomed regardless of how they try to change.