Hi deepcosmos, I`m not a teacher, but I`d like to offer my 2 cents since no one has responded to you yet...
1. He became a close friend and remained as such.
He became a close friend and remained so.
These sentences imply (to me) that this friend is no longer living. Unless you use "has":
He became a close friend and has remained as such.
He became a close friend and has remained so.
For sentence two just use "so" or "as such".
3. Such was not my intention. Using "such" sounds reasonable, but using "so" sounds strange to me. However, one never knows what a Brit or Aussie may come up with, and they may consider both to be acceptable . I`d simply say "That was not my intention".
Student or Learner