Not really. There is no leveling or loot, no procedural generation, each space is manually made. It has similar themes and tone to Diablo though, the original Diablo was certainly a source of inspiration.

No; it's a very shitty Heretic or similar.

You're better off going with Heretic or something as, even at 30 years old, it's done much better.

This game is much more like Metroid Prime than Diablo. You have a main hub area where you come back to upgrade everything, like so many games, but your method for unlocking new areas isn't entirely tied to what boss you fought or what switch you pulled. Like a good metroidvania type, paths are often unlocked by using new equipment in various ways to unlock branches in areas you've already been, possibly that have been shown to you outright but kept just out of reach until you have whatever it wants to get there from later down in the progression path.

Even combat has some subtle puzzle elements to it. One of the things the game doesn't tell you outright is that it remembers certain things over the long term that you wouldn't expect it to. Did you pick up and move an explosive barrel to some tactical location? Not only will it stay there your whole session, but even after you blow it up, forget about it, save, come back some other day, that barrel will spawn right back there. Health potions dropped by enemies will always be there later if you didn't pick them up. On the flip side, broken objects will stay broken in your current play session, but respawn the next time you load a save.

Graven handles enemy spawns in a fairly lazy but no less functional way. There are various invisible triggers and gates, an old method of handling entity spawning. However, again a matter of play session, typically the trigger will go off once, spawn enemies, or start a series of waves, and then not do it again until you load the game next. So, if you play in one long go, the game can get eerily vacant in places you've been spending a lot of time. On the other hand, if you only play in fifteen minute stints, you might not ever notice that mobs don't regularly respawn.

Diablo's strengths are in its ability to be a bit different through procedural level generation, and a bit the same by being a hack n' slash loop to pop all the little loot goblins between you and the next BIG loot goblin.

Graven is a puzzle box for you to turn over and nudge all the pieces in various ways, some that move the story forward and others that just suit your needs. The combat is there to keep things interesting between nudges.