SureAI

Alright, so I went and got the shovels for the Under Siege quest, and on returning to the gate, all the mages come running through after blasting a guard off of the gate. Cool. About a minute later, all the mages are dead. Time to turn in the quest, right? The gate guard doesn't think so, he'd rather keep screaming "Get to cover!" even though all the attackers are dead. I rested and came back to see if he'd gotten off of his adrenaline rush, but no such luck. Weirdest part is, attempting to walk through the unlocked gate onto the unoccupied road results in me getting harmlessly blasted back into the mine site by forces unknown, effectively preventing me from leaving.

I suspect I messed him up somehow by shooting out the window at one of the mages in order to provoke them for the soap quest, since he never left combat stance after that. What can I do here? Is there some kind of console command to make the guard chill out so I can talk to him properly?

you made no mistake in 'shooting' at a mage during soap quest... that is the proper action.

Even for myself the guard remained in cover for days... LOL.

However, i could still talk to him and was able to leave.

in your case there are only two options:
1. most likely not all mages are dead
there is probably one 'stuck' somewhere (perhaps not visible 'inside' landscape.)
This happens from time to time... NPC will get stuck inside rocks, trees, or walls. Oblivions engine issue.
If you can see the last remaining mage ... dispatch him. The guard may still be in cover but you wil be able to leave.
2. go to a previous save.
saddly this is possibly your only option.
With Oblivion and Nehrim 'save and save often' is very very very important.
SureAI has done an excellent job on the game... but there are many enherent faults with the Obilivion engine that can't be fixed.
Including 'getting stuck' and random game crashes.

This is not the 'quick fix' you may have wanted... but, it is good to learn early the Oblivion 'save often' requirement.
So far i have been able to circumvent the rare game issues by simply relaoding a previous save.

even with a few rare issues.... Nehrim is a must play game

Best of luck and happy hunting

Upload your save please!

"Save early, save often", eh? Like those old Sierra games but with less insane puzzles, I can dig it.

At any rate, I reloaded my quicksave which was fortunately still in the mines, and this time when the mages ran in a whole bunch of guards came in behind them and hacked them up. Nice of them to stop by the neighborhood! I think a nice healthy battery of saves should keep me from making Nehrim the longest mod ever by restarting every time the engine bugs out; not sure why I didn't think of that myself, so thanks for that.

We'll still like to have your buggy save to make sure this bug doesn't appear anymore (unless it is an engine bug).

just to be clear to all viewers...

'save and save often' is a Bethesda game engine requirement (nothing related to the work of SureAI)

Oblivion (plus all DLC), Fallout 3 (plus all DLC), and Fallout New Vegas all have issues with stuck NPC and random crashing.

(edited to remove 'offtopic' praise)