This got me interested at first, but now I'm not so sure. To begin with, I've never had the problems with the council that you describe. I can usually sidestep them well enough that they're not an issue (they don't even see me, so their hydras aren't a problem either). I agree that if you only have 30 points to put into cold, put them in Frozen Orb vs Blizzard.
I appreciate the safe nature of this, but as someone said, Sin traps are probably better at playing tower defense, with the added benefit of a Shadow Master and Mind Blast to help control crowds and tank with your merc. The big disadvantage there is the lack of teleport, but if the entire point is safe gameplay (e.g. HC) then you shouldn't blindly teleport anyway.
Ultimately, it turns out that Infinity can break most Lightning Immunes, so I'm not really seeing the benefit of dual tree designs for high end/twinked situations. I can see the benefits for untwinked and safe-playing situations though (but again, a Trapsin might be good/better for that ... you can buy cheap +3 claws from Larzuk). Thanks for the writeup. What gear do you use? You only talk about the charms. =)