The agent radius should match the radius of the collision capsule you are using for your character. To ensure you nav mesh always saves, goto Engine - Navigation Mesh under project settings and set Runtime Generation to Static or Dynamic Modifiers Only.
I don't know if this is a engine bug (I'm using 4.25) or it is how it works: I can change these parameters in my navmesh raycast and the navmesh looks like yours, nice and clean to access all maps part including building interiors. But when i close this level and return to it all the config i placed on Agent Radius, Cell height, etc; lose their config values and i can't enter inside buildings because nav mesh can't map the level again.
In games, elevators rarely move with the od exception here and there. Typically, when the doors to an elevator close they immediately start loading in another level using level streaming and once it's finished loading you can open the doors again. This is the technique I would recommend and use.
