As far as I could tell, it can't be installed offline or without a MS account. There's no offline installer. If a rewrite corrects this, that's a huge win
...Ah. Totally can-ish. I helped the RasPi4 community a few years ago with that. I'll go see if I can dig up those scripts. Though I have a sneaking suspicion you might be right in that to generate the initial flurry of download urls you need someone to have had an account to get the urls for version of the client you're downloading, plus the asset packs. Once you have those urls, you should be able to wget to stage/mirror to your hearts content locally, and then you just have to use a shellscript and a bonkers lontg java invocation that the launcher normally generates. You don't need the installer/an account to start the game once you have the clients downloaded, though not having to deal with wrangling lwjgl libs, java classpathery, and the occasional java runtime increment is definitely an increase in ergonomics, I'll give you that.
Didn't maintain it or anything, but another member of the community took the lessons learned from my teardown of it and I think polished it up. Needed a pick me up project at the time. Was around 1.14-ish I think? Also helped them get sound working in Forge.
Found one of the threads, though this either wasn't one of the ones with the gorey details, or they probably snipped the posts as they may not have been super comfortable with me laying out how to vivisect Microsoft's shiny new acquisition for people. I was in a rough headspace back then.