Gonna take it in small chapters her, as my time is limited:
1) Why not frostfire regalia
Too much stuff needs to be changed. Aftermath bonus is useless for the build, aftermath set bonus doesn't trigger often enough to be really usefull in the build.
Glasscanon counters the cheap/more accessible part of the build which is usefull for new players imo. (not saying it can't be good later, when you have the stuff you need for crafting)
Why HP only or a mix of HP/MP?
In my opinion(IMO) there are a lot of easier accessible "HP-items" requiring a minimum of crafting, which, are available earlier on, which can be good for the build, COMPARED to the mana items.
You CAN pick mana for defense only, but some/many of the mana items requires more crafting to suit the build or, at least to me, they doesn't drop as much.
Take for instance your chaos ring. Gives 25% hp - might as well use that, instead of bringing glass Canon, that counters your healthpool(your defense for now)
All said and done (and this goes for all the advice in this thread)
If you find an item that seems better than the one you had - try it out. You might sack some defense for more dps or visa-versa, but it might also be worth it.
Also - there are plenty other defensive options apart from HP/Mana shield, which will eventually be obsolete, but this is just a good/cheap option for beginners/early levels:)
To be continued...