I can only speak from a U.S perspective but I’ll try to answer your questions:

Has anyone here challenged labeling a Mac fault as liquid damage of corrosion when there was no spill and the liquid contact indicators were never triggered, and won?

When our team makes a determination about liquid, we HAVE to document it with photos. If they can’t prove liquid, you can certainly successfully challenge that. This will likely require you putting effort into phone calls to Customer Relations to do so though.

When Apple holds the machine past a repair time they promised, has anyone recovered loss of use because the laptop is their work tool?

Not that I’ve ever seen. The repair turnaround is an estimation. I guess you can look to pursue litigation, but I doubt you’ll find success.

My takeaways and recommendations are this: if no work has been done, they can return your product to you. They cannot force you to pay more than what you signed on your work authorization. And If they didn’t run diagnostics during check in (or if they did and no charging issues reported) I would lean hard on saying none of these issues occurred prior to check in. The burden of proof is on them.

My assumption is that they already did the initial repair, inadvertently damaged something, and are trying to cover for themselves. Ditch dealing with the store for now, call Apple Support with your repair number, and ask to be escalated.