I think a few things are not being considered.
Sizemore can clearly dig underground tunnels in the MK.
On page 112, Parson says to decrypt "three dozen troops, a few knights, and a warlord or two". I presume for city defense.
Parson then tells them to fan out among the garrison and find all the bodies ...and drag them all here.
Next time we see him he says "Yeah, just bring them in here. Stack them like cordwood."
So there should have been a ton of bodies, far more then the ones he ordered in the first round of decrypting, yet he ordered every body found brought to the portal. Granted a lot of bodies might have been lost in the fire, i don't think we know how body/corpse destruction works with disposal, but considering the city was an inferno and he didn't seem to think it would be a problem i presume some sort of remains to decrypt from remain.
Sizemore sealed himself inside the fortress in MK. We have seen that he could tunnel with very little notice from other casters unless they were literally standing right on top of him, and even then it took them awhile.
Think Parson has a basement filled with even more troops then we are seeing?
Fire rules on this page.
They had previously burned a Unaroyal city to the ground and then rode in and mass-decrypted the remains. So essentially, Parson has a lot of remains to work with.