FLEX AND DP

[mg43]

I recently participated in a PONY tournament with my 16U girls fastpitch team where I used the Flex and DP in my line-up. Of the teams we played against, not one opposing coach was familiar with these designations. I hope I used these "positions" correctly. My situation is that I have two girls who can only play first base (one lefty, one righty). I designate one as the DP and the other as the Flex. This way I can keep both in the game and substitute one for the other depending on the game situation, especially for batting. Up until now I have only flip-flopped these 2 players once per game by notifying the umpires and opposing coach of the substitution. I have 2 questions.

1. Is there a limit on the number of times that I can flip-flop these 2 particular players in one game between Flex and DP?

2. If my DP gets on base and I use a pinch runner off the bench for her, can she (the DP) re-enter the game in the next at bat or must the pinch runner become the DP?

Thanks for any advice you can offer.