Mostly just replying so I'm kept in the loop.
I'm preparing a proposal for and Eldoc enhancements that could link with this.
For example, i don't think that the eldoc function that displays in the echo area should piggy-back on the contents of the eldoc doc buffer used by the other display function. This was a misdesign (by me). Each display function should be given as much information as possible and then decide how and where to render without affecting other members of 'eldoc-display-functions'.
I think it's important to decide if things like line separation are properties of a given member of 'eldoc-display-function' or if eldoc in general. I strongly lean to the former.
João