There should be software that prepares yer computer to play HTML5 games offline or something. The average user cannot be bothered to go thru all that trouble. Also, browser independence it's something I'd love to see.
When you set your game to be playable offline doesn't it download to your C drive. It seems to do that with my game on the chrome store. You buy/install it and it downloads(like an exe), and then launches from a shortcut(like an exe).
I would say you have a much bigger concern with the amount of RAM your 100MB game will use. Because C2 puts everything in RAM all at once your game sprites become uncompressed to RAM. This means all those tiny .png files become gigantic 32bit BMP file into RAM. 100MB of compressed files could easily turn into 4 or 10 gigs of RAM.