If you struggle with inversion, you can sometimes add 'så' after a starting sub-clause to help your brain reset. 'Om det regnar, så stannar jag.' It's very common in speech!
Swedish grammar rules dictate that subordinate clauses have a fixed structure to distinguish them from main clauses.
Yes, it works just like 'because' in English.
A finite verb is the verb that is conjugated for tense (present or past). In 'Jag har inte ätit', 'har' is the finite verb.
In very formal or archaic Swedish, or in certain 'main-clause-like' sub-clauses in reported speech, it happens, but for B2, stick to the BIFF rule.
A finite verb is the verb that is conjugated for tense (present or past). In Jag har inte ätit, har is the finite verb, and ätit is the infinite form (supine).
No, only to 'sentence adverbials' (satsadverbial) like inte, kanske, alltid, ofta, ju, väl. It doesn't apply to place or manner adverbs like här or snabbt.