Characters will not only have friction tweaks, but will also have individual wavedash lengths which differ from their friction if necessary. For example, we want to keep Snake's DACUS length, but nerf his wavedash length because his ground game is so strong already and he'll have access to waveland item pickups.
We would like to, but that information is locked away in the modules (.rel files) and we currently do not have access to it.
Possible, but unlikely. You're free to replace any stages you like. In fact, we're going to be moving Project M to the /pm/ (instead of /pf/) folder, so you'll be able to use your custom content along with Project M.
As far as included stages, we're more likely going to go with the "+" series of stages (i.e., New Pork City+, etc.)
Item steal grab and diagonal item throw look very cool, actually. I would personally like to implement them but you'll have to wait on shanus' opinion. Worst comes to worst, I could probably code them for you and release them separately (assuming I have time and figure out how ).