First of all I agree that our job is to develop girls. And we try hard to do that (which makes this hard). At 12u, I'm trying to keep them playing softball longer term, and give them the tools and understanding of the game (physical, mental, strategy) so they can have fun playing beyond this team.
Girl A is a great kid and we love having her. We do play her as much as we can at catcher, but I'm starting to worry that we are placing her playing time as a priority to Girl B getting reps. Yes, Girl B is better, and yes she works hard on her game, and yes she can play anywhere (super athlete), but I'm wondering if reducing her innings at catcher (50/50 or worse) is helping her get better too.
I literally spend 1 hour before games, and tournaments trying to figure out how to get Girl A in the game (and yes DP happens a lot). We do have 1 other girl that can catch and gets 1-2 innings per tournament. In all honesty, Girl C is as good as Girl A, she just prefers to play OF. It is April and we still have a lot of softball left to play and a lot to still work on, so there is always hope.
It sounds to me like you are doing things pretty well for your situation. If/when Girl A's playing time gets questioned, be honest about the reasons, and clear about what your expectations are for things to change. Then, it's up to Girl A to meet those expectations if she wants a different result.