Zombie spawn/pathing on horde night + setting up in massive POI help

So me and my friends set up in the Navezgane Hospital. Its our favorite set up and we had it set up before the new update Thanos snapped our save. We have it pretty set up with two MAIN entrances. We built a whole wall arpund the courtyard because that was a common area they broke through. Yet they still get in the hospital from the back??

We checked though the wall and there wasnt much entry points that they made so not sure if they legit just spawn in the hospital or what. I remember hearing with POIs you need to destroy all the floors and replace them, is that still the case? Do I need to do it for every story?

We have two wide hallways as entrances with only wooden hatches to protect so I would assume they would choose the path of least resistance, but they still decide to slap up a concrete wall behind us.

Also do the zombies spawn based on where we are positioned. Like if we are in the front would they spawn directly there or behind us?