I've been using wild yeast for various beers, wines, etc since 2012
I've made tepache a few times and it will go alcoholic if you leave it too long. if the abv is 2-3+% then it's likely safe to consume (if you're overly concerned you can also check to confirm that the pH is below 4). However there probably won't be any residual sugar and it may not taste all that great.
yeah, you can back sweeten it, whether it tastes bad for other reasons is really going to depend on what sort of flavors the wild yeast and bacteria produced. Good luck!