I invite multiple people to review a PR since I don't know everybody's schedule and availability. But, I usually only wait for a single review to continue with the merge (unless I have to make updates, of course). There may be times that I definitely want a particular person to review something due to their expertise and experience. In that case, I will probably ping them directly to ensure that they saw the review request.
I think every PR should have at least one more set of eyes take a look, even for a typo. The requirement for a review/approver was discussed previously. And, I did find out it's doable. We just need to file a bug tracker with Eclipse to turn it on. I was going to wait until after 1.2 was done before pursuing this again, but I think we're close enough to 1.2 to move ahead.
The other half of the question is who should merge these reviewed/approved changes? My preference would be to let the submitter of the PR do the merge. But, if there is only one reviewer requested, then either the approver or the submitter can do the merge. If there are multiple reviewers, then the merge should probably be left up to the submitter since we don't know the intent of having multiple reviewers. And, of course, the submitter may not have privileges where s/he has to rely on somebody else to do the merge.
Or, to summarize, just use common sense... :-)
-- Kevin