1. Making a top down 2d game is fairly easy. It really depends on how you go about implementing it. One way is to use the character controller + tk2d tile maps for collisions, etc.
2. Yes, create a large tile map, and fill it as needed. The tile map max size out of the box is 1024x1024 tiles, but you can change that depending on what platform you're targeting, etc. Or if you want to code a bit, you can keep your tile map as a central window into your (infinite?) world and build chunks based on where the character moves. I.e. move the world rather than move the character.
1. There is no difference to the GPU whether your'e drawing 2D or 3D content. tk2d lets you optimise for your GPU in a few ways, so I can't see how this could be very different to any other engine.
2. We're working on a 4.3 update right now - it will support all of 4.3's features where it makes sense. 2D physics is already working.
3. No idea, perhaps someone else could comment on this.
4. Yes, you set up material overrides on the sprites, and the tile map just works with it.
5. tk2d doesn't do anything special here that Unity doesn't - you will have to figure out how to do this in Unity. Resources.Load / unloadUnusedAssets will cover most of it.