Grom would have never survived the battle with Mannoroth. It is inconceivable enough that two mortal orcs, while only one of them wielding magical powers that dwarfed to the ones Mannoroth possessed (As seen in the battle), single-handedly managed to defeat the greatest Pit Lord in existence with only ONE of them sacrificing his life.
If Grom haven't been bolder than Mannoroth expected, the Pit Lord would have slain them both with no real effort. Grom knew this, but he also knew the price, what makes him so great hero.
But even if he would have miraculously survived, your other claims are imposturous: As punwisp began to say; in the moment Mannoroth perished, the pact between the Burning Legion and the orcs was ultimately dissolved (excluding the Orcs in Outland, whom still remained faithful to the Legion's curse through Magtheridon.). As Thrall said, Mannoroth death freed all of them.
So, basically, Grom would have never consumed the skull, now that the orcs despised fel magic and now that Alliance and the Horde are allies (I believe Grom's hatred toward the humans would have decreased after Jaina assisted with his redemption).
Besides, I don’t think the Alliance and Horde had presence in Felwood during that time; the Legion and the Scourge had pushed their forces up to Hyjal's summit but it doesn't matter since Felwood had no real strategic significance to them. Even if they had presence there, Thrall would have never sacrificed his most talented warrior and best friend to protect a doomed stand; even the night elves were forced to retreat from Felwood in the end. So the skull was beyond Grom's reach even if he still lived during that time.
But since we are playing "what if…" let's go even farther: Grom insisted that he want to protect Felwood and went there with a miniscule force alone.
But unlike Mannoroth's blood, which granted the orcs physical power and stamina, the skull granted magical powers. Illidan could utilize the powers of the skull due to his advanced abilities and his understanding of magic. But Grom, and any other magically ungifted warrior, couldn't use it. Consuming the powers of the skull would perhaps grant Grom some magical power (Unlikely), but without knowing any spell how did he plan to use it?
One more thing is that Arthas came to Illidan knowing that he would claim the Skull of Gul'dan and destroy Tichondrius with its powers. But, unlike the arrogant and complacent Mannoroth, Tichondrius was very clever, cunning and powerful sorcerer, and Grom didn't have the abilities or the brain to defeat him even if the skull granted him powers he could use (as I mentioned earlier). So the Lich King would have never sent Arthas to Grom because he could never destroy Tichondrius. And without Arthas, Grom would have never come to know about the Skull and its powers.
There is much more, but I hope that's enough.
