There was a few posts a while back about this very subject, but I can't remember what type or polishing compound they used. You may try to do a search and find out that way, but the pictures of the polishing looked really good..............
Ya, I'd open them up and check them. If they sat for a while they could have some old fuel in them and they are sticking the floats or causing debris to stick to the float valve seat.............
Ya, they are ok for beginning to learn how to ride and they are pretty reliable. I have one for my daughter and two other family members have them. They are good bikes all in all...........