The problem with the make em reroll 6's power is you need to be fairly close to them don't you? With a lone slann that can't fly is pretty hard. I like the idea of a skink shield. Would that work to replace ethereal or would you need that as well? TLS - I've got the old OOP slann up on the shoulders of his mighty carriers. This is gonna be a double edge sword isn't it. From his eyes I can see over my units pretty easy which will also mean I'll be perfectly targetable by the enemy. Maybe the skink protection is extra worth it, gives a LOS roll too from some of the bad spells.
That's all well and good but don't forget about Warp Tokens the Grey Seer can take. In my last battle my opponent had five of them and used them all when he cast Dreaded 13th on my 23 Spear Saurus w/ Gor Rok. Even if I'd had a TG Slann unit with the item to disallow 6s he'd still have gotten it off... I'm not trying to be a jackass here, just pointing out a situation to be wary in. Not to mention if you give your Slann all those diciplines and magic items he's becoming VERY expensive and will become suseptable to fast cav/flyers and scouts.
They roll seperately for the warpstone used. If a one is rolled they take a wound but I think they can still get a ward save. At any rate my opponent the other day used five in one roll and only got a single one so if they use them sparingly they don't have much of a chance to worry about anything.