2: The stirling is simple to maintain as it is like any nelson valve but instead of the gas going through a powertube it is routed up through the bolt in the top tube - That is the only difference. When i had one i lubed the bolt after every use as well as the fake bolt thing in the bottom tube (The thing that grabs the hammer).
I also used to take the whole thing apart about once a month to check all the o-rings and lube up everything that moved to increase effiency.
3: Maybe some clear laqure spray paint?
5: The new one is just refined. I belive the one you own was made by a different company from the new ones. It is just a new design but is still an STP.