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For the people for whom it works, it works very well. I've had no trouble with it, even before the GOG release. I use the non-upscaled version, and have never seen the mouse problem.

There is no frame rate issue. The game runs at 20 turns/second, and always has. It was designed to work that way. The only time it goes faster is when it needs to catch up from spinning up the CD, at which point it runs very fast until it has caught up. This has gotten more than a few people killed by The Butcher when the CD finishes loading his "Ahhh Fresh Meat!" track.

The DEP issue is a bit annoying, but is a one time thing to bypass. I don't see any mention in this thread of Hellfire, other than your question, so I don't know what the Hellfire issue is.
https://github.com/diasurgical/devilutionX
here's the solution to all your problems, if something doesn't work properly, you can report an issue and it will get fixed ;)