1) I'm pretty sure that you have to wait until the next phase of the beta.
2) I think everyone needs a copy. Eventually Impulse will be used as the multiplayer client, although I don't think they have announced when that will be happening.
3) It may sound like there are "only" 8 heroes but there is some variety in the skills you choose that makes them different. They also plan to release a few new Demigods in free updates soon after release. I am almost 100% sure that there will be no hero builder but modders will probably end up making more Demigods. I don't think there will really be a problem with the number of heroes though. The different team combinations will be enough.
4) As far as I know, there will be no mods allowed in ranked, but custom games can have mods. There probably won't be big support for it for a while but there is already a community-made mod-manager, and the Supcom engine (which Demigod is based on) was pretty moddable.
5) Yes, you can leave spots empty.
6) They aren't as good as humans, of course, but they use items and spells and heal and etc. They will hopefully be better by release; right now I think they just want the game to work.