Much of these are non-issues if you are in a Multiboxing guild that is going for doing all Raids possible. Those kinds of guilds will be leading the raid and deciding on allowances.

In the Americas there are both Alliance and Horde guilds who have goals for raiding. Currently the Horde guild is large enough to readily fill a 40man raid and the at this point the Alliance side could only field enough to farm 20man trash so is looking to grow. All in all this is still a little early as most teams are still leveling to try to get to cap let alone actually be at pre-raid BiS gear levels ready with consumables and professions to raid...

My understanding is that the Horde side has already looked at recruiting some non-boxers for some key roles like main Tanks. There are also some leaders that deem ranged casters, particularly Mages, to be more desirable and asking to see if any boxers with non-mage teams would be willing to roll a 2nd team of Mages (and maybe get some help in the process). I don't know that many people at this point were jumping at the idea of leveling a 2nd team (most are still working on their 1st team) and instead offering to play one or more characters from their main team (once they got their team to cap & fully prepared).

I doubt that mixing melee and ranged by a single boxer is a good idea. My inclination is that you should be doing one or the other and your slaves should ideally be essentially what the main foreground characters is doing. At least in the beginning I generally wouldn't recommend mixing roles even of the same damage type (like being the main melee tank and running a bunch of melee DPS at the same time).

Personally I will likely try to work up to doing 3-4 melee DPS (all Warriors all doing essentially the same thing) with at most a dedicated Healer (Paladin) running with them at melee range. The Paladin would be giving an aura & blessing the melee/himself & judging the boss with 'Blessing of Light' to give the main tank, all the melee, & himself healing upon hit. I personally will likely do raids on my single main character (not boxing) with a full time raiding guild and mainly try to work in extra DPS on raids we have on farm and/or have speed runs on to try to catch-up gear new members.