I'm far from being wise but here goes.
it would appear you have a shop demo unit and would like to turn it into a standard retail unit (Cex). if thats the case then i believe these are the answers.
1 after you convert to retail unit if you update to any official fw above 3.55 you would permanently have a retail unit.
3 probably best only using 3.55 or below (cfw or ofw) just incase you want to revert back to demo.
4 does the serial number have any importance.
5 if use rogero latest cfw psn is available.
im not 100% sure on these answer, but im sure some of the smart people here will correct this if wrong.