Some people struggle too hard to make logical narrative sense out of "lyrical" lyrics. And/or fixate in a futile way on a line or phrase that may just be a fluke - a misremembering or mispronunciation of the original, a whole verse pulled in unthinkingly from another song, an undigested blot of mustard, a crumb of cheese.

Having said that ... I first heard Earl Hines' heartfelt rendition of SJI as a teenager, and, ever since, I've been satisfied to understand the "Let her go" verse as a kind of passing, not clearly-defined, fantasy - the speaker imagining the soul of his dead lover wandering the world, but in the end, returning to her one true love (him!) - or, if not returning, at least somehow realizing that she could not be happy alive or dead with anyone else. If that's braggadoccio, it's of a very familiar kind: no one loves you as much as I do; we were meant for each other, etc. And it seems to be natural to fantasize about what the dear departed are up to: read any obituary column (or thread).

As for the "When I die" verse - it is common, if not universal, to contemplate one's own mortality upon the death of a loved one. Even to start thinking about one's own funeral arrangements. In this instance, it might not be too much of a stretch to suppose that the speaker feels he has nothing left to live for, now that his "baby" is gone, and that a showy funeral is all he has to look forward to. Again, nothing unusual in such sentiments.

So, the only narrative we get out of all this is: a guy has just seen the corpse of his lover; he tries to accept her death and "move on", but still acknowledges a strong connection with her; he contemplates his own mortality, perhaps feeling little desire to live, and tries to come to terms with his despair and eventual death by imagining an impressive funeral.

Not sure why anyone would want to bring murder, syphilis, or even brutally egocentric emotion into it.