First of all, thank you so much for the kind words and I'm really sorry for the late response 🙇♂️
We designed the zone to essentially mimic critical successes from DnD, so the fact they occur randomly is intended. That said, manual zone sounds fun! Allowing it once per campaign might result in some players keeping their one time use until very late in the campaign though.
To be honest we didn't even think about an NPC or encounter tables! Can't think of any reason why it wouldn't work. We'll see if we can make it happen in the hopefully not so distant future.
Although there isn't a formal rule set that allows players to play as trainers (yet), you can absolutely still do it with the caveat that trainers only roleplay. Another good way to ease the burden off the GM would be to suggest to your players to be part of the same team. Or at least not have every player be in a separate team with different trainers.
I really appreciate your post and feedback! We aren't capable of expanding the team right now, but we'll 100% keep working on both UMA RPG and other projects.