Hahaha, he wants to tank with his warrior and is seriously entertaining any notion at all of doing so with this raid? That's so nieve. Tanks have to form or be the first person to jump on a raid in order to secure a spot. You can't "jump on a raid" by being level 75 and casually gaining experience. A flat out "No" to Avrus' warrior is certainly in your raid's best interest.
But that brings up some issues with Avrus. Now, he's a Whiskey raider who won't need any loot (maybe one item early on) and will soon get bored who is also annoyed and bitter when his warrior hits 80 and you start sharding the tanking drops. I don't believe I would make him a regular- he'll probably wander off in frustration after a short while. Worse, the "Friends with Stonebelly" hook is just a strong possiblity of Stone deciding to go ret when Avrus' warrior hits 80 and the warrior/Stone "swapping roles." That would set you behind for like a month, non-optimally double up on prot warriors, and potentially lead to the loss of 2 raiders instead of one.
Be very up front with him. It's always possible that I'm searching for ways things could fail, and that Avrus is responsible enough to make up his own mind and has a firm grasp on what is really in the raid's best interest. If you're still concerned after chatting with him, there is always the diplomatic "We want to give a raid option to folks who aren't regulars in Whiskey already. so more guildies who want to can have the chance to raid." It only rings a little hallow with the (surprising and annoying) lack of response you've gotten.
As for Marg, of course. It's only fair to him and your raid both to make your own opinions of him. He's disc when he heals, so be certain that you use a mod that estimates his shield abilities or his numbers will look worse than they already are. We've already demonstrated that a situationally aware raid fight where the healers cannot cover for eachother like Twin Valks is a great proving ground.