The problem with that is that the automatic scanners will detect second class stamps - and automatically send it by second class post. Although you will have paid enough for it to go first class, the machinery won't realise that, and you will just have wasted your money.

Yes.. not a problem.

some times I post my mail or a parcel at a post office. depending on the weight, I watched them use few differently priced stamps on the parcel , it's quite normal because different weights require different pricing .. post office don't have the exact priced stamp so they use differently priced stamps to cover the cost..

Yes, because that adds up to more than the first class price, but you'll be paying too much. It's only 9p more for first class.