Most other varmints tend to go higher, try to dig through attic vents and the like.

You're going to want to get an exterminator out to find out where the little bastard got in. You'll want that plugged up or said varmint will invite the whole family in.

end of the vent/chimney/whatever to see if that is how the critter got in. It seems more likely that a critter would enter the end where the air is blown out rather than enter through the side of the tube (which would mean chewing through it or it rusting or rotting through)... Often the end has a screen or flap or both -- which would be more fragile and susceptible to violation than the tube itself.

We had one somehow get in the backside of our dishwasher last winter. He chewed up all the wiring.

It’s a good rat repellent.

During pandemic, I got some of those full face covers that sit on glass frames. I would recommend at least that, but full cover (sides, top, bottom) with a soft rubber gasket on your eyes would be better. They aren't real expensive. And a nose/mouth cover.

DO NOT mix ammonia with any other fluid, especially bleach, which creates a very dangerous gas. Seriously, it will put you in ER, hospital, or morgue.