LG P760, insufficient disk space, help please
a) Yes, you can use your external as an internal storage. (Requires Root access. Using link2sd. I don't know any other app) 

b) Yes, it is possible to unroot your device. (http://www.wikihow.com/Unroot-Android)

C) I haven't tried that. But, yes you can do it.

I think that you want to avoid using link2sd app because of the risk.So I suggest you move your apps from Internal memory to an External memory(if you have any).
By rooting your phone your warranty will be void, plus you take a huge risk.(But if anything goes wrong, you are not alone. So you'll always find a solution on net :p )
This is the biggest disadvantage. But on the bright side there are plenty of things you can enjoy.