Why "我想了退这件新买的毛衣。” is wrong?
我想退了这件新买的毛衣。

Semantically, 我想 itself connotes a completion already because when you say it you already think about it.  So we don't need 了 after 想 for completion.  That's why the sentence in title is not correct.
退了 denotes I want the action 退 get done. &quot;Done&quot; is for completion, so we need a 了 after 退.
P. S.  I somehow agree with you Chinese language is easier to learn if you focus more on logic, convention and culture, especially for second language learners.  Chinese grammar is the hardest part of the language. Only few people truly master Chinese grammar. Second language learners can easily create a wrong/stilted sentence based on the grammar rules they learned.