1. Is it known roughly what % of the world's population has been abducted?
I don't know if we know
for sure as that's going to be a little difficult to verify, but maybe we can deduce? If roughly half the population are OPs (whom there is no benefit in abducting), the the other half then are highly likely to have been subject to abduction. So, 50% give or take?
2. Do the majority of abductees have no conscious memory of what happened to them?
If the above is true and half the population are subject to abduction, the majority certainly seem not to remember it!
3. If someone has used recreational drugs for a prolonged period of time, is it safe to say that they've probably been abducted?
No I don't think so, the person can be an OP. Taking drugs doesn't qualify one for special treatment, perhaps even the opposite is true, that taking drugs makes one less likely to be abducted. Why bother messing with a subject that is quite happy messing with themselves?
4. Is it in one's best interests to find out if they've been abducted, if they suspect they have?
I don't know that it would change anything. What might be more useful is to grow your level awareness around the subject, reading Laura’s ‘High Strangeness
’ would be a good place to start. Knowledge protects.