The size of the 12.9" keyboard is ideal. I'll deal with the extra room on the sides. And the cost differential is minimal. It won't be my primary case, just my primary keyboard.
You could try and if it doesn't work then return it. I'm not certain if the magnets will line up or not. If not, then it's not going to be very stable to type on.
I can't see why not, though it'll make for a very sloppy solution. For all the compromises, I would have thought that if you want a full size keyboard with the 9.7in, you'd be better with a smart keyboard and separate case; it's a better keyboard, has the shortcut keys, and will save you a few quid.
The size of the 12.9" keyboard is ideal. I'll deal with the extra room on the sides. And the cost differential is minimal. It won't be my primary case, just my primary keyboard.