Yes I know why you have so the interfaces, but as far as I know: Linux simply can't do what you want. So if you want to access PiAlert from your main PC on .6.X, you need to make that accessible from .6.Y on that VM. If you want to have the management port (UI) only open on the management interface, you would need to remove it's interface on .6.X.
As I said, as far as I'm aware Linux simply can't not route packets properly in an environment like that. I won't respect that the interface packets came in on needs to also be the outgoing interface for the return trip. I also had that problem and eventually j I've just given up.