First, the logic for the number. Assume that only 1 player buys a Curser or Looter, with the given amount in the supply and assuming they are given on each play, each player would receive 10 Curses or 10 Ruins.
For the FPA, that has been discussed a lot in numerous places and the extra turn has been postulated in multiple versions most notable being the "ghost Province" wherein players can take an extra turn and buy ghost Provinces even if they aren't in the supply. Of course there are other FPA apart from Victory points which include earlier access to playing attack cards which can help snowball an advantage (playing Militia first or putting a Curse in deck before a reshuffle)
The inherent FPA is present, there is no denying it. That is why the DXV solution is elegantly brilliant. Play another game, whoever lost, goes first. The tough part of balancing the FPA is that it is different in different kinds of games and much of the strategy of playing Dominion comes down to end-game decisions. Any sort of FPA "fix" would mess up the precise nature that the best players manipulate the end-game for their advantage. What would be lost upon this would be a loss of 3-pile manipulation and strategic duchy dancing.