Shit. Wasn't thinking correctly. Super busy and distracted at work.
He can only Counterspell so much, yes, but he can cast to block spells with physical barriers, too.
The Ring of Spell-Battle can be used whenever in the round it is most needed. The next/first/whatever can be part of a readied action, and Celerity can be used for a third(DP has confirmed that Celerity can be used even if another Immediate Action or Swift Action has been used, this has come up before).
Before using Celerity, though, Viserys can use the Amazing Initiative + Mythic Power + Belt of Battle combo to cast again. It can't be a readied action to Counter a spell, though. Could still be used to raise a Wall of Mithral, Prismatic Wall, Wall of Suppression, etc. in a pinch.
Gotta go. Will edit plan when I have another free minute.