I installed the game using the offline installer version 1.41a_(41462) and played for a few minutes. The mouse works just fine on my end. Opening/closing doors, picking up items etc. and moving around in general - everything worked as intended.

That's what I've already written here. But not everything worked as intended! Hard Mode is still broken!
A fix for that was finished almost 2 weeks ago, but still no update here!!! I ask me what the helll happened here with GOG and Frictional Games! Somewhere people don't do their jobs properly.

Yep, the suggestion to revert to the 1.4 build fixed the mouse entirely! Odd, that--usually updates improve a game as opposed to breaking it! Looks and plays fantastic with the remastered mod! Gives this game a new lease on life!

There seems to be an update 1.41b. Anybody tried it yet?

I just launched the game after installing the new patch. Hard mode is now available (see screenshot) and the mouse works just fine (see video).

https://streamable.com/9z2wku

Mouse seems to work fine 1.41b