Chip, sorry this happened! Janet and Al have given great posts! You seem to be resigned to getting a new liner (unfortunately that is probably a MUST at this point ) if you get a pool co. to do this have them pressure test the lines while the pool is empty. If you plan on doing the liner replacement yourself, I can give you lots of advice/ help and others will probably 'chime in' and we can tell you how to pressure test the lines It may well be as Al said, the multiport is syphoning - we'd need to know more about the height of the pool vs. the filter system to make a guess at that.

As for the walls of the pool itself being harmed/ damaged, if the pool was installed properly you have a low risk of that Winter is all but gone (thankfully) but you should have enough time to get the pool back in shape so you can enjoy it all swim season

Please let us know how everything works out