A: Ever get an award for penmanship ? - sarcastic, implies "Your writing is bad."
B: Guessing you think I should have ? - fairly neutral reply, perhaps asking: "What's the problem?"
A: This "P" looks like an "R"... Could have unraveled your PMI warrant big time.
This could have made your (search?) warrant invalid in a big way, that is to say completely illegal.
B: Came all the way down here to rail on my cursive ? - "Did you come all the way down here to complain about my writing?"
Big-time - in a major way.
Rail - complain, rant
Cursive - handwriting.