I think you're exaggerating the hatred between Dwarves and Elves on an individual level. If a Dwarf and Elf fought side by side for a while and went through the same things that Legolas and Gimli went through, and the two of them are relatively open minded (and we've seen a bunch of Dwarves like that so I don't see why not Elves) then I can see them being friends. The Grudge of the Beard was avenged thousands of years ago and the Eonir are practically unknown compared to Asrai and Asur, so it would allow the Dwarves to form a fresh opinion. The opinion would be informed by prejudice from long held beliefs, but Gimli and Legolas had that about them in the beginning until they learned to get along.
Secondly, Karag Dum is literally Mount Doom. It's the perfect mountain for the whole thing. It would, however, be an extremely uncomfortable topic for Dwarves, so Mathilde would not write about it.
I think Aragorn and Boromir being Kislevites would be fun. Mathilde doesn't have nearly enough experience with Kislev to make a "return of the Boyar" type plot though.
Also, I trust Mathilde to make Fellowship of the Ring explicitly gay instead of having it be vague enough that thousands of gay people project onto the characters and have to make their own stories to explore themselves. Gimli/Legolas and Frodo/Sam were vital parts of my self discovery, and I hold them dear for it.