Do realize Nintendo, along with all console manufacturers, imposes certain standards and restrictions on developers for retail 'box' games. Wii/3DSware is pretty much "anything goes" because they're begging for interest in that sector.
That and; A) Konami execs are tards; B) "Forget what you know about Castlevania"; C) LoS was highly profitable, and in 3D
False. They don't restricts games from being 2D or 3D. There's a lot of 2D games on 3DS already, retail even.
The other reasons you listed have nothing to do with the game being or not 3D or 2D. And, considering there's a console game coming (and in 3D), I don't see how this cannot be made in 2D. I don't care if it's not pixel art (it would be better), I just care about the gameplay being that of a sidescroller Castlevania.
This game will probably be a spin-off of the new mythos they are trying to stablish.