OK--Jeremiah, I can help you out here. Sorry if it seems like I was handwaving, too much. In the end, it really does all make sense...I'm just trying not to get bogged down in minutiae.
You killed Syvattagor in the morning--we'll say it happened at 10am, although I wasn't calculating. We'll say you encounter the black orcs an hour later, thus giving you a reasonable amount of time to have had the OOC and IC discussions that we've had thus far, since Syvattagor's death. So, it's 11am when the Black Orcs arrive. Let's say it'll take you a half-hour to talk to them, tell them the plan, etc. So, it's 11:30 before you can start to move back to Hyrwl.
It takes about 6 hours for a small group of dedicated people to get from Oceanus to Hyrwl. If the army marches at a relatively leisurely pace, it would still only take them between 7 and 7.5 hours. So if they leave after Syvattagor disappears (I'll be generous, and say that it takes them an hour to regroup, forward elements could arrive by the rough equivalent of 5pm, but might be as late as 6:30. Ghouls don't get tired, and the draconids are well-disciplined. Estimate short, just in case. It's mid-autumn, so there's no way this battle will begin any later than nightfall.
Since the flying mounts are ULTRA-encumbered, it'll take them more than a little while to get the approx. 30 miles from where you are to Hyrwl. On foot, hussling, you guys could get there by 5. Since the mounts are so encumbered, I'll say the travel time is approximately 2/3s of that it would be, normally (that *is* a bit of DM handwaving). You make the 30 miles in 4 hours, which gets you to Hyrwl (assuming you leave at 11:30am) 3:30.
Essentially, from this point, you have (in-character) between an hour and a half and 3 hours to do whatever preparations you need, all while the city girds itself for war. As much as possible, I am going to try to condense these preparations, so that the battle can be fought (or at least started), tonight.
Remember, when Xath grabbed Syvattagor, they were already ready to march. He was in the midst of a "win one for the gipper!" speech when she grabbed him. As I have noted individually to a couple people, although Syvattagor's absence has slowed them down, these aren't mongol hordes. They don't have to go home when the Khan dies to pick a new one.
Hope that helps!