1. Yea storage spaces is the best way to go on windows 10.
2. It will just show up when you move it to anouther compatabile system(so windows 10)
3. You can just add more drives to the pool and then expand the virtual disk.
3a. Storage spaces isn't a traditional raid so you can mix sizes. THis depeds on the exact config, but by defualt its 1 parity for every 2 data chucks, so with 2+2+3+1 you should get about 2.3 tb usable
4. Under folder properties, then sharing you can share the folder for others to use