Even more from same dev...
Wow, I think you guys are going to quickly overwhelm my ability to answer all of your questions!
Answers after the questions:
Are mercenaries ignored if you do a coin-split with the /split command?
Yes.
Also, how aggressively do they follow you around? I already have a sprit-dog riding my leg and getting in the way when I'm trying to loot. Will I now also have a mercenary standing directly in front of me?
Mercenary movement is very context dependent. When you're not in combat, however, they follow you similar to the way pets do. The distances are easily tweaked if they become an issue. In combat, they try to position themselves in good tactical positions that will depend on their type and what stance they are in.
Does the tanking mercenary do more than just taunt to hold agro? Or does it just taunt like a pet would and leave it at that?
It will do more than just taunt.
If a mercenary kills a mob after you die, will the mob's corpse poof like it does if a DoT kills the mob after you die?
I suspect there will be some cases where the corpse may poof if you did not contribute to the fight, to prevent you from going AFK and letting your mercenary grind XP for you.
Will a mercenary sit down to med? (And more to the point, can you prevent that since sitting is a sure way to die in some zones).
Yes, caster mercenaries sit down to med. You can prevent them from doing this by putting them in the "Do nothing" stance (all mercenaries have this stance).
For the healer mercenary, can you give as an idea of how effectively it heals? What I mean is, will it generally use fast heals whenever your health reaches a certain threshold or will it blindly chain-cast CH?
This greatly depends on the situation and quality level of the mercenary, but in general it will try to cast the most efficient heal it has that will land before it estimates you will die.
Can a mercenary change its mind and interupt its own casting to cast a different spell?
Yes. This comes up fairly often if you get bursty mob damage with the healer AI.
Will the mercenary complain if I choose to send him to certain death without buffs while I execute a "Brave, Brave Sir Robin" manouver if I do a bad pull?
You can't really send him anywhere. Mercenaries currently will not attack mobs without the mob being aggro on the group. This could require some skill for some of the more squishy classes, but then, most of the squishy classes can solo pretty well already (soloing being the main situation where this would matter).
Will (future non-tank and non-healer) mercenaries understand the concept of root-rotting? I'd sure hate to get a necro mercenary to help with dotting and find out that it prefers nuking or some-such. I guess this questions could be better worded as "can there be a dotting only version of a caster DPS mercenary if you happen to make a caster DPS mercenary".
The sky is the limit as far as making AIs. I'm sure we'll have fun making them for quite a while as long as they are popular.
RE: Requests for a "fair share" payment method--we have discussed this as a possibility, but we didn't think it would be very popular. If during beta we get feedback to the contrary we can re-visit this idea. It is something that could take a fair amount of coding time to do correctly though, so I'm not sure at this point we could fit it in even if we decided we wanted the option. I'm certainly interested in hearing more feedback on it though. That case of fighting the named mob and having the mercenary ninja the cool loot is something that we felt would leave a sour taste in player's mouths. Now, of course, they could simply dismiss the mercenary before fighthing the named, but perhaps they wouldn't be able to take the named without it. Yeah, there's a lot of things to think about with that particular solution. How to make it non-exploitable could be very tricky.