What to do with failed meringue mixture? My best thought is using it in pancakes. Would that work? And how would I got about substituting that?

Failed, as in collapsed?
Yes you could use it in most cooked dishes where lift or height is not essential...your pancake idea, also waffles, noodle casseroles or bread puddings.
How to substitute...choose a recipe that uses approx the same # of eggs and volume of sugar, then eliminate those from the recipe as you make it this time. Probably mix dries & wets, then combine and cook.