Dark_art_: The way I understand it Proton is just a marketing name for a bunch of open source tools, the most proeminent being DXVK that runs over wine.
Why would I need a marketing name to runs games?
I mean, you can use standalone Wine or being lazy like me and use Bottles or Lutris, all 3 options can use GEproton.
This is what I have understood so far:
There are many (hacky) fixes and improvements that don't make it to Wine from Valve's Proton. Wine is for all Windows applications, Proton is specifically for games, so it has that benefit alongside being able to incorporate game-specific fixes quickly. Because they don't care how hacky those fixes are.
You're right, you don't need Proton or GE-Proton - but it can get things working earlier than Wine (staging or especially base vanilla), sometimes with new features. Cutscenes in many games were a big one for me, could only see them with GE/Proton for many games. Things like FSR and DXVK-NVAPI came quickly to GE's variants too, as new features. Etc.
The thing is, you're not supposed use Proton outside of Steam. Not in Lutris, Heroic, or Bottles. GloriousEggroll has repeatedly stated this, with many reasons for doing so.
UMU however enables leveraging Proton's fixes and features outside of Steam, where Steam Runtime doesn't exist. That, and the fact that the underlying UMU database will minimise work for the devs - at least, no more duplication of work across the board, people doing the same thing in different places all the time.
Bottom line, when using Lutris/Heroic (Bottles?), the integration should be nigh invisible and seamless, and you get all that good stuff handed to you. You don't need to manually use umu with a dozen environment variables to mimic the good work of Lutris and Heroic devs.
Dark_art_: Is the UMU launcher just a replacement for ProtonUp-QT (tool to inject pGEproton on Steam)?
UMU is not a launcher. It's something that launchers use to run Proton in a similar way than on Steam, with its Runtime, to utilise it's gamespecific fixes and other features. Lutris and Heroic will have (currently experimental) integrations for it. You just select Proton for a Wine version/runner, and that should be it.
Protonup-Qt is just a Wine/DXVK/VKD3D downloader, it doesn't actually do anything except queries for new versions of different runners/softwares and allows you to download them into the right folders.
Don't get me wrong, it's extremely convenient and I use it too, but it doesn't do anything else. Lutris and Heroic have their own downloaders too, though more limited in versions you can obtain.