[ajuc]

There are rules to devoicings, it's just that nobody bothers to remember them because you know what sounds good and what doesn't "by ear".

Sz/cz etc are just your regular dyphtongs, like English sh/ch/th etc - they are perfectly regular and phonetic. The only tricky one is rz, but it's also regular - there's a rule saying when you read it as ż and when you read it as sz depending on the preceding letter. And there's like 3 exceptions when you read it as separate letters - in words like "tarzan" :)

As for regional differences - all versions are correct usually. People in Kraków even accent on the first syllable and read "trzy" as "czy" :)