What would it look like inside Canterlot Castle if Iron Hoof's hold on the Royal Guard became ever more shaky? Could the Guards start disobeying orders and start muttering about him in empty corners of the Castle? I should not expect them to disobey him, as they hate him, but stick to their duty. Could Iron Hoof use more bluff, threats and bullying to keep them in line?
Will Celestia see the mess, and pass the duty of over-seeing and running the Royal Guard to Luna? Or rotate it between themselfs? Shooting Star could be persuaded to come back down. Or the Captaincy could be passed to a unicorn noble.

I think it would depend on Shining Armor's inside stallions. If we're talking pre-empire split and they're found by sympathetic soldiers you could end up with a lot of soldiers defecting. If they're not found by Shining's soldiers then we may see them shammed and removed with Iron's cronies taking their places. The ones tossed out may head to the crystal empire to appeal to Shining or end up on the streets. Talking post, it is likely that Celestia will see, but as far as what she will do we can't predict. We know she upholds the order, but not if she'll go to war over it.

4498158 I'd be surprised if it doesn't get to that point. Things are definitely getting worse, and there's only so long they can put up with Ironhoof, who gives off the most visible "Entitled to have you" vibes I've ever seen.

Another thing that could be done post would be to take a partial line from Ann Rynd (especially if Celesta supresses the real reason for the empire's split) 'Who is the Alicorn of love?' *shot*

Now we have learned Iron Hoof "has generously offered to fill the post himself", here are a few problems. One is; if he is guarding Twilight all day, work in his office which needs doing will not get done. What may this work be? armour supplies or armour polish, health insurance for the Guards, etc. If this work does not get done, Iron hoof will fall behind in his administration dutys. This could hamper the standing of the Guard. But, as Celestia Herself said: The Guard has weathered worse.