They don't grant spells like that of a cleric, but they
do have warlocks, whom they imbue with magic. Warlocks don't have to prepare, research, or otherwise acquire spells (except for those with the book, when they come across rituals). The warlocks just
know the magic, and some of it they can cast at will (via invocations), which is something that
gods generally don't or can't let their clerics do. Can gods imbue their clerics with magic in the same way arch-whatevers imbue their warlocks? Who knows?
Also, take a look at the three types of quasi-deities--all of them were gods or were the offspring/direct creation of gods. None of them were mortals who ascended to godhood, even though that's a thing that happens in D&D. It also doesn't say that things that aren't gods--but as powerful as gods--can't grant spells.
Maybe. We haven't seen their statblocks yet.

But no, there's no reason for Bane to go after a god with a different portfolio, unless he's just greedy to increase his portfolio for some reason.
Of course, the other thing that separates gods from arch-whatevers is avatars. I seem to recall rules a few editions ago that indicated that the greater the god, the more avatars they can make. Can an arch-devil make any avatars? If he can't, then Asmodeus could possibly kill one of Bane's avatars, but he'd likely be actually injured in the process, whereas Bane could pop out another avatar soon, if not immediately (as that page in the DMG says, killing a greater god's avatar has no effect on the god itself). If arch-whatevers
can make avatars, they are probably limited to a very small number of them--maybe one at a time--with a lengthy recharge time. Years, decades, even centuries between avatars, perhaps. At most, archdevil Asmodeus would be able to
clone himself. And even if he has a thousand
clones, Bane can still produce more avatars.