I would not consider this machine done just because the keyboard is broken. It may require a replacement, but that should not cost too much, depending on the model. I would recommend you take your machine to a trustworthy repair shop to get an estimate and diagnosis. I would say it is time to replace the machine when it no longer does the things you need it to do without getting in your way or being an issue. If you can still do the things you want it to do, I think you should stick to it a bit longer.

I use mine with a ThinkPad USB C dock, which works just fine. I also have a workstation with a Dell USB-C dock with ethernet and it works fine with M1 Pro. I would recommend to look for used business docks. I have seen some Elgato or ThinkPad docks for below £100.

Nothing much you can do about it. Over time the keys collect oils from your fingers. For some people it’s not as much of an issue but if you eat while using your computer, rarely wash your hands or have sweaty hands, this will happen quicker. Turn the machine off, get a microfiber cloth and some eucalyptus oil and dapen the cloth with it and wipe the keys. Make sure the cloth isn’t wet so that any moisture gets into the keyboard area.

Eventually the machine should limit charging to 80% by itself to preserve the battery. You could also use an App called Al Dente to do the same. If you regularly use the full power of the machine, go with the permanent 140W charger, if you only use the machine for office tasks and rarely demand high performance of the machine, then go with the USB-C charging.

If you have a 15" model it is worth a bit less, somewhere around 500 - 700 USD but if you have a 16" model in good condition, I would say somewhere between 600 and 850 is a decent deal. https://www.ebay.com/sch/i.html?_from=R40&_nkw=macbook+pro+2019+i9&_sacat=0&LH_Sold=1&LH_Complete=1&rt=nc&LH_BIN=1

Short of buying a defective model and stealing the stabilisers I don’t think you can buy them anywehre, since they are not an individual part. However I know what you can make them yourself with a paper clip or equivalently thick wire. The typing feel also may not be much worse if you use them without the stabilser.

It may be possible that during this time the battery may have discharged below the save minimum voltage and the battery management may prevent the battery from charging. Nothing much that can be done. If the battery indeed is in need of replacement, it could swell and become a fire hazard.

Go with Windows. While Parallels offers Windows on Mac, it is the ARM version of Windows, which has to emulate x86 apps, and compatibility is just not there yet. I assume most specialised software will not work under Windows on Arm.