Note that in post #2 I said "perform a smash and grab before the defenders could mount a defense".
The idea is to keep moving forward and not get bogged down in reinforcements, which is precisely what the PCs didn't do.
Neither the forces outside or inside are a match for the party, or even half of the party. But exactly how they split up their forces really depends on how things are going from moment to moment.
What changes would you recommend to make it a "fair" challenge though?
The players hypothesis is that I put them in an impossible scenario, and my hypothesis is that it is perfectly winnnabl, they just went about it in the wrong way (and had some cold dice).
I don't know how you would test either of those hypothesis while changing the scenario.
Especially when you consider that going into it they already had full information of the scenario thanks to Krystal's scouting and Feur's time manipulation.