There are also, seriously, darkspawn actually popping out of the ground. There's no reason to believe that they all came pouring out of the tower.
Because they are under the tower. Go play RtO, exit the tunnels, and look up. That's what "under" means.
To see if they do connect.
No. Just no. Play RtO. That's literally the most absurd conspiracy theory I've read about Loghain.
Because they're not connected to the tower. It's also entirely possible that the ogre tore any barricade to shreds when entering.
Also no, because, as previously stated by yours truly, Loghain's field of the battle was obscured. He needed a high point. Uldred points it out because he's a mage and can shoot a signal from anywhere. The Chantry priestess is the one who doesn't want Uldred blasting magic around (because she's an idiot), and Cailan is the one who insists on the Wardens lighting the signal fire. And yet this is somehow Loghain's fault..... ??????
It seems that you've spent so much time hating Loghain that you haven't thought through whether your ideas actually make sense. He did enough wrong in-game. You don't have to make up new things.
https://youtu.be/Svb7xgYkBd8?t=23m54s
The guard confirms they came from the tower's lower chambers, not just from "underground". There were no holes in the ground you walk on to get to the tower. They all came from the tower, and as I wrote, the tower is completely overrun, which means they had easy access. The darkspawn in the tower were ahead of the rest of the hoard. Alistair confirms this. Loghain must have purposely left the lower chambers open.
But under doesn't mean part of. The Tower Guard states they are the tower's lower chambers. If there was no hole in the tower, then they would be exploring caves near the battlefield, not the tower's lower chambers.
But you're claiming there was no hole until the ogre came through. So how could there be lower chambers to the tower if there was no hole? And why would the tower be off limits if they were just investigating a cave near the battlefield? Your theory makes no sense.
I don't write that as a matter of fact, but it wouldn't surprise me if it were true. More likely the Tower Guard simply didn't notice that hole because he didn't go into that room, but what is certain is that Loghain knew about the tunnel leading to the tower and intentionally left it open so the tower would be overrun and the beacon would not be lit.
But there would be some debris from the barricade. And which is it, the tunnel wasn't connected or was there a barricade? Because why barricade a tunnel that doesn't connect to the tower? As stated above, Alistair confirms that the darkspawn that took the tower were well ahead of the horde, almost as if they knew the plan.
It wasn't obscured from the locations I mentioned. He could have picked any of those locations for the signal, but he picked the unsecured tower so that the beacon would not be lit. He does not support Uldred's suggestion because he knows the tower will not be lit anyway. So there's no reason to put Uldred's reputation at stake. It's Loghain's fault for picking the tower and leaving it unsecure so that the signal would not be lit.
Loghain planned to have the battle at Ostagar lost even before it began. It was always his intention to kill Cailan off so that he could claim the regency. His plans started before Ostagar. He plotted with Howe to get rid of the Couslands. He plotted with Uldred to ally the circle to his cause. He plotted with Jowan to poison Eamon. This all happened before Ostagar. So of course the Tower of Ishal was a ruse to cover his own retreat.