Yeah the shortened versions are southern informal language.
Then why is this course teaching us the informal imperfect tense, rather than teaching us the actual words behind the shortened versions?
What you're calling "the actual words" are actually shortened versions themselves: do't ti from doeddet ti from nid oeddet ti, and you could extend it either end (more formal or more informal). So what course creators do is look at all the forms available (about five in this case) and choose the one most appropriate for their learners.
You're not causing problems. I'm the same when I'm learning a language - I want to know where it all comes from. That kind of thing helps me remember stuff. Just making you aware that the "shortened versions" are still actual normal everyday words, and the "actual words" you mentioned are shortened versions in themselves! ☺️
I understand, and I'm sorry if it seems like I'm causing problems. I just want to understand the language.
The new verb conjugation table in this section really helps, by the way.