1) Neither. It is still a +1 enhancement bonus, that provides a +4 untyped bonus to attack and damage rolls when used to sneak attack.
2) If the GM says yes, certainly. Deconstructing the sword of subtlety suggests that the "subtlety" part is a +20,000 gp flat cost (the other option is that it's a +2 special ability with an additional 4,000 gp flat cost, which would be weird). The cost of adding additional abilities is the difference between the cost of the sword with the additional ability, minus the cost of the base sword of subtlety.