[quote:8n61c3k8]-How well does C99/C2 handle having a game take place on a single incredibly large, non-linear 'playfield' rather than having it chunked up into levels?
They can handle it but like any other game creator the performance may bog down if there are too many objects at one time. A solution would be to destroy objects far from the screen, and create them as they get close, thus reducing the total objects to be processed at any given time. It would be a similar solution for most if not all other game creation programs, as it would be pretty game specific.
[quote:8n61c3k8]-How well does C99/C2 handle a playfield that isn't easily broken down into squares/rectangles?
They should do fine, objects can be positioned anywhere and be of any shape, they aren't restricted to a grid.
[quote:8n61c3k8]-How is making a Sonic-type engine easier in Construct than in MMF2/Game Maker 8.1, if both of those systems already have full-featured, free-to-use Sonic-type engines available for use?
The process is basically the same, but I've found Construct to be much easier to code with than MMF or Gamemaker, both of which I have tried in the past. If you haven't done so already I recommend doing some tests in all 3 programs (MMF,Gamemaker and Construct) and make your own determination which is easier to use.