Nope... I used the Rockstar Nexus' Downgrader and it crashes everytime I want to downgrade and if I want to start the game (with the remastering mods installed from this downgrader) the game tells me that the game is unknown and an error from the open limiter adjuster that it can not detect the version of the game and then the game won't start.
I used to have this mod installed in my game and it works pretty muchly fine! Does your game have any other mods installed?
I know what the issue is, because I had exact same problem you had last year.
Like you, I have the European copy of GTA SA retail disc version 2, and the Nexus Downgrader just would not work with it, kept getting UNKOWN GAME error. So I gave up with it, bought SA on Steam and downgraded it with Nexus, this time it worked perfectly. But I don’t want to play a modded SA and depend on Steam(which updates SA all the time and sometimes messes up the mods if you need to reinstall), and it’s only now I realised that for a retail disc version to be downgraded by Nexus, it has to be a US v2, that’s what the mod states. I have just ordered a US v2 retail disc copy for myself in case I get more issues with the Steam version, but if you don’t want to do that, you can try buying it from Steam, it’s cheap.
For now, just buy the Steam version and downgrade it with Nexus, as its much easier. But you must create a copy of the downgraded game in a new file and play SA from there as a non-Steam game, then you uninstall the Steam version.