Help me choose a realm to prepare for WoD + community events

[Lax]

I don't know about everyone else, but if I do play in WOD, it will be solely dependent on if multiboxing functionalities are enabled in Ashran. I also wonder if it will be possible to to cross-realm events with the zone with groups.

Ashran is a world PVP zone and we have no indication that they plan to disable follow anywhere other than battleground instances. Arenas and world PVP (including Ashran) all have follow. Ashran is also supposed to be cross-realm, but I can't really tell you if there's "other raid groups" for it for example..

Been awhile since I played WoW, is there no cross-realm support? What about cross-faction guilds?

Some realms are connected. http://wowpedia.org/Connected_Realms

In order to participate in wargame tournaments you must be targetable, so connected realms and cross-faction are OK there.