Really can't speak for Atilla (yes its stand-alone, don't need Rome2) or Arena sorry.

Yes there is a new Rome2 campaign coming, pretty much confirmed to be early expansion of Rome within Italy.
Apparently Rome2 has had better sales/more ongoing players -> is getting patches/expansions while Attila is tapped out.

Yes Steam will download latest version & automatically patch.
Current beta patch is opt-in & CA have used the Steam beta options to allow access to older patches since they have broken compatibility with older mods. (there was about 3yrs between patch 17 & new patch 18)

Honestly the battle AI isn't especially scary or at least not as long as you've done your work on the Campaign level.
Its probably better at keeping formation than Med1 etc, it maintains a line, tries to get on your flanks with cavalry, holds back missile units etc.
At least initially anyway.