How did you come to the conclusion that PUCYKOWICZ is a rare Polish surname and originally from the Krosno area?
The distribution of the name in Poland. See this map:
Of course, these people may have settled there after the war, coming from what is now the Ukraine. Only documentation will reveal what the case was.
I think if you spoke Polish and were Catholic that made you a Pole.
Essentially yes. That is what polonification means.
But can you be Orthodox and be Polish...? Historically?
Yes, that is possible, and it did happen, though historically people who were Orthodox were less likely to identify as Poles, unless they were Russified Poles. It's actually pretty complicated and depended on the historical period in question.
It seems languages and religions influenced people's national identity somewhat.
Very much so. Ethnicity as we understand it now is a relatively recent development, from the Romantic Era. For Ruthenians and Ukrainians, the idea of ethnicity didn't catch on until much later. Most people just considered themselves "tutejsi", or "people from around these parts". And others had no problems with considering themselves members of more than one ethnic group.