I'd be a bit scared of moving to Poland, too, especially because I am very not-Catholic and I hear folks over there find imperfect Polish incomprehensible, rather than merely annoying.
No idea, how poles stand to immigration. It's pretty uncommon there and the few immigrants you get there, are mostly from other slavic countries.
I don't think being atheist would be a great issue. Most of the younger generations you meet in churches there, treat it as a social meetup. As long as you don't start a theistic argument with some of the older people there, you should be fine.
Totally. The stereotypical god-fearing Pole doesn't exist anymore, apart from some isolated strongholds, which are naturally dying off. What remains here, is a huge, devoid of deeper meaning, inert load of religion(i.e.Catholicism) themed cultural habits.
From what little I've seen of the US, you guys appear to be worse in that respect.
As for immigrants - we're mostly prejudiced against our eastern slavic neighbours, in a similar fashion as our western germanic ones are prejudiced against us. Everybody knows they're thieves, brutes, and hobos.
Other than that, it's a nice place for foreigners. Just choose a large city, so that you'll get more English-speaking people.
It's also very easy to get a job in a language school, if you're a native speaker. Especially if you've got some experience already.
Also, I'd say immigration is not that uncommon. In Kraków, it's hard to pass a day without at least once hearing somebody speaking some foreign language - and that's outside of the holiday season.