1a and 2a are OK, but not great. A native might say them, but probably wouldn't write them. 3a is WRONG. If you use "for" or "since" to talk about duration, you HAVE TO use a perfect tense. It sounds very strange if you don't. EDIT: After further consideration, you COULD say something like "Tomorrow, I will sing for 20 minutes". There, the entire 20 minutes occur within "tomorrow". Similarly, you can do similar things with other tenses "I swim for 20 minutes every day" "When I was young, I went hiking for two days every week" "I went hiking for a month in 2015". However, "20 years" is longer than "next week", so this interpretation wouldn't make sense, so 3a is simply wrong.

I mostly agree. 3a is definitely wrong. 1a is ok, but 1 sounds better. 2a is perfectly correct if you haven't started reading the book yet. If you have already started reading the book, 2 is better.