"Perpetually" allows players to permanently "change the characteristics of a cardno matter where it goes afterwards." The effect of Davriel's Withering will not leave a creature, even if it dies and returns to the board at a later stage
Put them together and you get the best two-drop milling spell ever
As soon as Decimator Beetles ability resolves, state-based actions are checked, and the game sees that you have both counter types on your Servant, so one +1/+1 counter and one -1/-1 counter are removed at the same time (resulting in Servant dying)
Volcarona can be obtained from Mew pack in the Mythical Island set
If you still have questions about how double-faced cards work, check out the Double-Faced Card Rules Page
according to PSAs pricing data, at its peak, this card fetched over $8k