treat present-perfect as indefinite-past(past without a definite time point/interval).

Grammarians duty is to explain the phenomena(usage), but not to tell some usage is is misusage/bad.

What do you mean by grammar translation? Most of the grammars use the terminology of classical grammars(grammars of Classical languages, like greek, latin etc)

having been begun

In your sentence, "having begun before 1975" is modifying "years", leading to absurd modification.

restrictive clause defines its antecedent. Ex: The book that is on the table. There may be many books in the room. But the restrictive clause *restricts* the reference to the book on the table. Non-restrictive clause provides additional

interjection

may = possible, might = more doubtful than "may"

I have already done so. I have not yet done so. In your sentence, "yet" in the place of "already" is preferred.

"in grabbing" is fine if the success of an action is preferred.