Learn English – “Could not happen” vs. “could have not happened”

Either the the or the of would make a difference. Concerning the: For the whole of last year means all of the preceding calendar year, whereas for the whole of the last year means all of the preceding 365 (or 366, in this case) days.

For the whole the last year is ungrammatical, and for the whole last year sounds quite colloquial, though it's not incorrect. I can see the whole last year meaning either the last calendar year or the last 365/366 days, though I would say that it's more generally used to refer to the last 365/366 days.

"Which could have" in this case would indicate that the impact is still to come.

If the study has concluded then "Could have had" would be much more appropriate.