I'm at the boss battle with TK in the underground, I have on inf health, ammo, survivor health, durable weapons, and slippery.
Tk walks to the gate and closes it, and then the boss fight starts. Now, from reading the guide and seeing the fight on youtube mercenaries are supposed to spawn. I even reloaded the save without the trainer and they still don't spawn.
I had the same problem myself and a same kind of problem with the first zombrex case. The mobs wont spawn due to the clock being modified. First turn your clock back to the time it's supposed to be - in this case, its after 7pm first night. Go out, kill some zombies (gain a level, I got one while fooling around abit while testing) and go back to the underground and there will be those mercs. Hope this helps.