Let’s pretend that none of this had ever/never happened.
Let’s pretend that none of this have ever/never happened.
Both sentences are in present tense, and the noun phrase, 'none of this', is singular, hence 'has' could be more suitable than 'had' or 'have'.
For information, 'none of these' would be plural.
Also, in both examples, the adverb, 'never', if used would lead to double-negative.
Both examples hence are not recommended.
A guide on the use of 'none of' is shown below.
We use none with of before the, demonstratives (this, that), possessives (my, your) or pronouns:
None of his old friends knew what had happened to him.
It doesn’t matter. None of it was your fault.
We don’t use none of when there is already a negative word (not, n’t) in the clause:
She doesn’t remember any of us.
Not: She doesn’t remember none of us.