1, MAYBE, but it would probably be a PITA and you would definitely void any warranty.
2, the U series aren't anything really like the desktop series processors;
i5 4200U - 2 cores, 4 threads. 1.6ghz - ~2.3ghz
17 4578U - 2 cores, 4 threads. 3ghz - ~3.4ghz
So whilst you're gaining clock speed, you aren't getting any more threads, so there will be a performance limit.
3, I don't know where you'll get an i7 4578U without purchasing another laptop to salvage.