"He could not have been selected, were it not for his friends."
Yes, I believe this sentence is the most correct of all.
Some readers may wonder why it's "were it not", and not "had it not been". I think this is because it's usually more natural to use the Past Simple in such contexts. Especially, when we speak about things that are still valid, as opposed to things that happened once e.g.:
"He could have been eaten by sharks, had it not been for a ship that rescued him"
"He could have been eaten by sharks, were it not for his amazing swimming skills"
"He could not have been selected, were it not for his friends."
Yes, I believe this sentence is the most correct of all.
Some readers may wonder why it's "were it not", and not "had it not been". I think this is because it's usually more natural to use the Past Simple in such contexts. Especially, when we speak about things that are still valid, as opposed to things that happened once e.g.:
"He could have been eaten by sharks, had it not been for a ship that rescued him"
"He could have been eaten by sharks, were it not for his amazing swimming skills"
Do you agree?