Under sink PITA

So it started with a bit, then a lot, of water running out on the garage floor (directly below the kitchen) when we ran the dishwasher. Finally opened up the wall to expose the drainpipe and found the problem. The black ABS pipe, right at the 90 degree bend heading down to the white PVC, has a substantial crack. At first I was thinking I could Bubba-it with a good epoxy. But I want a solution that will outlive me before I seal up the wall again. Also need to add more insulation since what was there got saturated.

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I wrapped repair tape around the crack and verified that was 95% of the problem. But it wasn't the whole problem! I still felt moisture coming down from somewhere on the back side of the drain pipe to the right. Was afraid I'd have to cut open more of the wall to find another crack. I'd checked under the sink before and saw/felt nothing. Upon deeper inspection, the reverse osmosis system had a tiny leak where I couldn't see it and barely feel it. Replaced filters but real culprit was old O-rings that had hardened and flattened.

This RO system (Culligan) was installed poorly almost two decades ago. It's like the guy just tossed it under the kitchen sink without really fitting it to the space with excessive tubing length and not hanging the manifold so it just flops around. Bigger issue is the weight of the 2 gallon tank caused the thin plywood floor to sag well over 1". So I had to cut out a large hole in the plywood floor and block it up to support the weight and then toss a scrap of plywood overtop. Works but looks like crap and still isn't flat (other water stain is from a waste disposal that leaked and had to replaced a few years ago). What would be a good water-resistant material that I could lay over top of the existing floor? I'll have to cut it to shape, probably in two pieces, to get it in and fit around pipes. Open to other solutions but replacing the floor isn't a realistic option.

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And how concerned should I be about that hot water pipe with all the calcium leakage? Is that a ticking time bomb? Thanks!