I use the slim jims for water changes, usually around 15 gallons which puts it at 75% full. It definitely bows out on the sides but I would think it can hold 20g, maybe not for long multi days though before it starts permanently deforming.
Possibly, but I would be a tad weary of any containers with too many folds built into them, ones that they use for stiffness, because they're meant to hold garbage (with a LOT of air), you put water in there the air component goes away and they bulge outward probably under stresses they were not designed for. As Casey said, temporary usage only
I have been keeping one or two of the 24G next to my tank for top-off water for at least ten years - filled to within 1" of the top (ok - on occasion filled well over the top). They do bow when filled, but other than that no issues. I originally kept two in a special - built credenza that was tight enough to prevent the bowing, I tore down the credenza when I started my forever-delayed basement ATO project, but as of today, one Rubbermaid still sits in the dining room.
I suspect that over time they will deteriorate and fail, so I replace them every couple of years.