Sounds like the "right" thing to do is to trash your current liner, re-form and re-compact the dirt, and replace the liner. Suggest shaping the walls of the pond like steps (probably 1 to 2 given the current depth of you pond). Steps and top edge of pond should be horizontal so rocks don't keep sliding into water.

I would stay away from the pre-formed tubs. They will severely limit your filtration options (and are usually too small for anything but the smallest of fish). Use a fish-safe EPDM rubber pond liner instead.

As far as the garage goes, that may be the domain of an engineer or building contractor.

Thanks for the advice! We thought of reducing the depth of the pond (from 32" to 24"), getting a 20-30º slope on the walls, and cutting one step for marginals, which ought to give us some latitude for re-shaping. Hopefully the slope from the bottom will support the bulging, which is happening higher up toward the center of the walls. My husband wants to use some stone dust in there when we add fill to re-shape. Would you suggest some kind of mechanical compaction?

I'd be concerned about reducing pond depth. Here in NJ we dig at least 3' to avoid a solid freeze. I think stone dust would be a poor idea - it compacts wells but doesn't "stick" well - meaning your walls may begin crumbling before you even get the liner back in. Check out the soil thats there (once you start digging). Sandy soil will not hold. Clay will be hard to dig, but will hold strong. For small areas use a hand tamper, larger areas may need a mechanical one (what is large is up to your own stamina). Just be sure to compact every 1-2" of new soil - don't add all the dirt at once and expect to compact it all the way through.

OK, that is really good to know. Thanks so much. Will go with the EPDM liner as well--first we have to get a contractor in to look at the garage corner...