Quote Originally Posted by Yuki View Post
Question 1

Не прочитав письмо от мамы, он ушёл.

Is there any possibility that he had read some part of the letter before he left?
No, there is no hints in this sentences that he ever started reading
If you want to specify that he dropped reading in the middle it will be
Не дочитав письмо от мамы, он ушёл.

Quote Originally Posted by Yuki View Post
Question 2

Нельзя + Imperfective such as Вам нельзя заходить туда.

I think this generally means "prohibition". Can it mean "impossibility" as well?
No, it means "prohibition"


"Prohibition" or "impossibility", depending on context/intonation will be
Вы не можете туда зайти