Boundary?

It's more likely that there's an infinite size field filled with junk and we can only see a spherical shape chunk of it.

I interpret this as the "boundary" being one-way away from us, with us at the "top of the potential well". As opposed to the more commonly proposed idea that the universe is inside a black hole.

Hawking radiation is based on the curvature of the event horizon, so in this case I think it should balance negatively, sucking out energy from the universe, probably by releasing negative energy particles in the particle perspective.

This doesn't make physical sense, which matches that such an event horizon shape also doesn't make sense and shouldn't be able to be created I think.

That's a real big "if", but sure if that's true then yes it's a possibility. But there's no evidence for that scenario as far as I'm aware.

If we were inside a singularity then the event horizon would be well outside the boundaries of our universe.

As for us evaporating via hawking radiation it would depend on the laws of physics outside our universe.