Unfortunately last time I tried both tear and midori had some showstopper issues like lack of support for authentication. Are there any other free software options?

Upstream modest (and tinymail) is a dead end (last commit: June 2010), and in many ways more buggy than the Diablo one (just ask Fremantle users). The Harmattan replacement turned out to be both quite bad and closed source so porting it is out of the question. Claws-mail does still work fairly well though.

What's wrong with the built-in telepathy stack?

Qt is huge, not really an option for Diablo devices unless you don't want to install much else or are willing to clone the rootfs to a card.