God, in my opinion, always makes a way, for His ministry to be done. If the issue is musical, it is relatively easy to feedback, I always encourage folks to grow and come back and try out as they work on the areas that might not be workable right now. We also offer training in various areas to help folks get some of the basic skills for this, although, for worhsip team level stuff we don't do that much. I ask the person if they want honest feedback musically, whether they are asked to join the team or not. That way, if they know they ned to work on maintaining pitch, it doesn't have to be a mystery to them any longer.
Worship is the most important thing, absolutely.
No questions there. This is one of the most crucial things. Talk to your pastor before assigning someone to the worhsip team. That will help immensely. I look for how the person fits relationally.