Every power in WoW can expand your hitbox and fill your bags with shit people will want to kill you over because of the needs of the game, but there's a respectable level of variance for something that was obviously not intended to be consistent at all. The warlock point is one I've gone over at a few points but to whit - that the warlock himself doesn't physically mutate doesn't affect that there's a price being paid. Gul'dan only turned green despite being the most powerful regular warlock we know up until he transformed himself, but what he does still comes with a price tag in terms of the environment and people. There's a physical cost attached to fel by default whereas with the void provided you have the relevant personal qualities you can change. Both can turn you into something raidable, mutate you either bodily or by soul but the mechanics and effect on the world around you varies and that variance has been consistently portrayed.
There is a Freudian element with the effect it had on his dad that's primary to his character. That's why I emphasize how he viewed his father drinking the blood because at the end of the day Garrosh had no real relationship with his father. It's all projection and he impresses the qualities he and by extension the ideal orc has to have onto his dad and assumes he already has them. And what he appends to Grom is that Grom wanted to prove his ability to control the blood's effects so others didn't have to bear it and that because of the nature of the power he couldn't do it and was corrupted despite being, to Garrosh, the ideal orc. This helps shape his view of the power but it also tells us what Garrosh values, which is something controllable and which emphasizes the qualities of the person using it. Also, while we're on the topic of personal backstory to lend to his overall positions, the dude was in Outland a place that's a shithole in large part due to fel use and later was among the orcs who are the direct descendants if not just literally the same people who used fel magic.Anyway, as to Garrosh, I think the core issue is that he talks about his father and the old Horde far too much for that not to seem like his biggest motivation, not just as to the Heart but in literally everything he does. He's Daddy Issues: The Character and is hardly ever shown musing on the specifics of what power is acceptable and what isn't while he spends every waking moment overcompensating for the failures of these aforementioned people and institutions, it's the drive of his entire character. I guess our difference is mostly one of interpretation.
The 'only a minority of orcs' thing aside I think it helps to remember that the canonical majority of these tards later backed Sylvanas, so them actually being extremely committed to racial inclusivity when waging indiscriminate war is among the more compelling readings of their otherwise incomprehensible motive and moral code. Your point about the Mag'har is exactly what I'm getting at - they do mistrust all cosmic forces and associate the Light and fel because the two subvert their will without a chance for pushback. The main difference between the void and light which both are based on willpower is that the void's willpower is entirely personal, if you control yourself and push it towards your goals you can do it. While the Light requires allegiance to an abstract creed, so it'd be more suffocating. Their view and Garrosh's in that case are analogous, but it's a view that the MU orcs don't have since they come from a different cultural mileau. In turn, Garrosh entertaining not just void magic but also machines, dinosaurs and whatever other retardation Blizzard found cool at the time speaks to his overall lack of attachment to any power in particular and more towards what power can be bent consequence-free towards the achievement of his political objective of benefitting orcs who agree with him. That the second he had opportunity he killed all warlocks and how later all the groups he even slightly influenced like the Iron Horde and Mag'har didn't entertain fel as well pushes to that point. As does that the warlocks were extremely unpopular even under a Horde that was majority drunk on fel and had to rule from the shadows and Orgrim was quite popular for promptly killing all of them.Plus I doubt his view is THE traditional Orcish one. Desecrating the elements certainly would be horrifying enough to a lot of them (even the Dark Shaman themselves lament their actions upon death) and most Orcs canonically didn't back him up; sure, their chief issue might have been that they didn't want to become Nazi Orcs due to how bad it looks on a resume but I don't see many of them looking at the spooky scary tentacle monster power and going "hell ya I want more of those!" not too long after being freed from spooky scary space demons. And the AU!Mag'har seem to despise, or at least distrust, any and all cosmic influence in the first place, judging by female Thrall's reaction in the Visions and generally how they wanna be free of corruption. And to me that's a reasonable PoV considering even the Light has skeletons in its closet, and everyone else is a good bit worse except for Life which they have been subject to the one negative instance of, the Everbloom.