Bearded Dialogue Master

I'll agree he's an idiot, there is no denying that, but killing off Sombra just to make sure Twilight was ready is not an excuss when he could have used some sort an illusion of a giant distastor that just happen to result in all other options for Twilight to not be ready to vanish

Speaking of other options when you realize that Sombra was the only villain smart enough to let them win to let the guard down so he could destroy those options it makes sense. He was the only one with the power to destroy the Tree, not even Discord and Tirek could do that. Discord using Sombra was a good choice, but still he could have used an fake monster to do the deed instead. Ressurecting Sombra only added more dangers since he knew more about Grogar than any of them, even Discord since it took Sombra mentioning the name "King of Monsters" for Discord to show off that little lie.

Actaully now that I think about it took Chrysalis doubting him and Tirek to be used as an example before they could trust that he was the real Grogar. So the others did hold a better place in the plan if the writers had took more than an episode to come up with this twist. Really rewatching the season after the fact makes it very clearly that this twist was made on the fly. Literally every moment Discord and Grogar are on screen up until the episode greatly conflicts each other. Sure someone could say double spell, but that's really an excuse for any twist. With that kind of plot point it could have very well been Celestia in disguise and the twist would still make sense. The show itself clearly was building up to Grogar as the true final villain, but than a brain fart happened and they decided to change it.

Under that context there was no plan, just us making excuses for a poor attempted at a rushed twist.