In answer to your first question, the settings are relative to the player's historical performance. To use SBs as an example, if a player was 1 for 1 in SB attempts in real life, setting him to 5 is not going to turn him into Rickey Henderson; it just means he will try to steal more than he did in RL.
The answer to Q2 is the same, I believe - since failed sac attempts are not recorded as such, just the gross number of sacs come into play. So, a player with a lot of sacs is considered a good sacrificer regardless of the actual percentage of successful sacs he had.