In standard modern English (both BrE and AmE) 'weren't' is one syllable and it rhymes with 'burnt'. An English-speaking child who is still struggling with their spelling might write 'weren't' as 'wurnt', because that's how it "sounds". There are accents where it approaches two syllables, though. I don't know about N.America, but I do know that many Scots accents pronounce 'weren't' as two sounds. The Scots pronunciation could be transcribed as 'wurrunt'.

You only get one written form: weren't. There is no other written form of the word.

What is NAm? I would not lean either Wernt or Wer-rent as neither is English. You are looking for "weren't".

I’ve definitely heard the pronunciation variant that you refer to as wer-rent. It’s not common and I feel like it only comes out in certain situations. I think it may be more common in certain accents as well but I cant say for sure without some research. My inclination is that I’ve heard this in the southern American accent in something like “if you werrent so...”

In the UK weren't is pronounced "wernt". I don't know anyone who makes two syllables of weren't (wer-rent), however the degree to which the "r" is sounded can vary so you might get more of a "werrnt" with some accents.