I've also seen the bug Cheesypoofs saw - if you don't all get to the last saferoom alive then need to restart the finale then the dead people don't respawn on any following attempt. I've no idea if it's fixable & I've seen it with other maps (Blood Hospital is an example) BUT you could provide a spawn room for dead players near the start of the level as a workaround, or a spawn room at the finale itself (as is "standard" in the Valve map finales). Then it's not so critical that a player doesn't spawn in the safehouse.

Like the campaign :) All the places for the zombies to lurk in the trees make it a decent challenge!

Well, it's simple. Just make a brush of the door. Then tie it to entity func_door, adjust it's parameters, then make an input to it from the button OnPressed - Open