I've done some testing with this, the only way to get mending is with spawn egg villagers. Even when you do get eggs, they're pretty rare, it takes forever sometimes for them to have mending.
I like this idea, as someone who's never stepped foot in the lps arena, teaming seems like a pretty big issue in lps. Pretty much invalidates the intention of competition and fairness, and this seems like a perfect way to mitigate that.
I feel like some sort of repair system is kinda necessary. I like the fact that your idea uses mana bc like you said everyone can obtain it easily. +1 from me