Here are my answers (BrE):
1) I'd like to know which tense we can use (in academic English and not in everyday speech) with the time conjunction "since". Le simple past is unanimously chosen but what about the present perfect?
a) It is / has been a long time since I SAW her. 100% correct
b) It is / has been a long time since I HAVE SEEN her. (also academic ?)
c) It's been a week since I SAW my friends / I GRADUATED.
— All these forms seem fine to me; the present perfect in each case keeps one eye on the past; the simple present simply presents a fact.
d) It's been a week since I HAVE SEEN my friends / I HAVE GRADUATED.
— friends, yes, as you can see them at different times during a given period; but graduating, perhaps not, as you only graduate once.
e) I've been cheating on you since I KNEW you. (?)
f) I've been cheating on you since I HAVE KNOWN you.
— I would add "ever" before "since".
g) How long is it since you KNEW her ? (??)
— This is a little strange; if he has stopped knowing her, he won't know how long it is since he knew her!
h) How long is it since you HAVE KNOWN her?
— I would change "since" to "that".
i) How long HAVE you KNOWN her ?
2) Is it true that we cannot use the present tense in the main clause when "since" expresses time but can only use it when "since" introduces a reason? However, let's look at the following sentences, can't they be made of time clauses?
a) She LOOKS quite different since her illness. (time clause ?)
— Sounds ok to me, though I would probably use the present perfect instead of the simple present. Time.
b) He IS SHOOTING the most expensive film since Star Wars. (correct ?)
— Sounds ok to me. Time.
c) My mother LOOKS younger since she dyed her hair. (time or reason clause ?)
— as 2a. Time.
d) He LOOKS much younger since he shaved off his beard. (time or reason)
— as 2a. Time.
e) She no longer VISITS since she got married. (time or reason?)
— Ok. Time. Or "She doesn't visit us, since she got married."
<...At some point I thought that the present tense could be used with the time expression "since" only with state verbs but "visits" above is a verb of process, so what is the correct answer?...>
Sometimes sentences that sounded fine on a Tuesday seem very strange on a Wednesday. I'll look at them again tomorrow and see if I still agree with myself, before answering!
All the best,
Student or Learner