Like any other Windows program, the installer has created a group Fsdreamteam in your Windows start group, with a link to the manual.
Am I reading it correctly that reverting back to the previous existing GSX environment is only possible via a restore from system backup--in other words, uninstalling the new GSX and reinstalling the old version with an archived GSX v1 installer will not work due to the live update now serving files for the new version?
If you want to return back to the old GSX, it's enough to run the old installer with the network connection disabled.
One particular question I have is whether it's necessary to replace *all* default jetways at once--can just a few default jetways at a particular airport be excluded and replaced with SODE GSX jetways, or is it an all-or-nothing proposition?
No, you can keep the default jetways if you want, but then you'll have to create single exclude files for each airport, just like as if it was a 3rd party airport with CTRL+J custom jetways. See the attached screenshot: I just restored the default jetways, and replaced only ONE, at a default airport.