first time pool owner in florida, pool was losing water when running main drain and skimmer-isolated it to main drain and starting digging. Found leaking joint where main drain goes down. So im currently shocking pool to get clear enough to see the bottom and running just the skimmer. Then i was hoping to plug main drain with garbage bag and cut a repair vertical joint going to main drain. Should this work!!!!
The success of repairs like you're attempting depend greatly on (a) your skill and (b) the quality of the specific materials you use and (c) the quality of your application.
If you are talking about cutting down the wall, to allow you to run a new pipe, you should be aware that cutting the re-bar will greatly compromise the structural integrity of your pool. In stable sandy soil, this *might* not be a problem, but I wouldn't do it myself.