Questions will be answered! Misconceptions shall be splintered! A Queue day, a red day, ere the sun rises!
Since this is a different timeline with no impact on our own history, I'm wondering why our characters should even care, story-wise, about these people who are historical characters in name only. That isn't Grom, that isn't Gul'dan, it isn't even really Draenor.
The only redeeming thing I can think of is a Legion invasion at the end of WoD that destroys the fragmented timeline and sets up the next expansion. Have it so we can visit our Garrisons through the Timewalkers so they weren't for nothing.
But yeah. Since they're so big on stressing these AREN'T famous characters from lore, why does it matter who they are at all?
Well, there's several reasons. The first is pragmatic. The Iron Horde is coming through into our world, blowing up our settlements, and making war on us. So we should care about these people because these people are literally trying to murder us. I don't know about you, but I tend to be concerned when people want to kill me.
From another perspective, it's like this - yes, that Grom isn't the Grom who died fighting Mannoroth to lift the blood curse. But he is Grom. He shares many life experiences in common with the Grom we know - he has as much right to be called Grom Hellscream as the other one did. Saying that's not really Gul'dan is foolishness. It is Gul'dan. It's not the Gul'dan whose skull ended up in Felwood, allowing Illidan to consume its power and become a demon. But it is Gul'dan. And the reason we shouldn't just ignore that fact is simple - even though his life has taken a different path than our Gul'dan did, he's still the same evil power hungry maniac, just as the Grom we'll meet in Draenor is still the bloodthirst orc whose howl drove fear into the hearts of others. They're not identical to the people we know, but they are similar, and that's a source of knowledge about them. We'd be fools to not use that knowledge to our own advantage.
Up to you if your character is a fool, of course.