Can reviewers ask questions of or communicate with the author outside of the formal review process?
In general, I would consider it unethical for a reviewer to deliberately communicate with an author outside of the formal review communication channels.
The reason for this is that the person ultimately in charge of the decision is not the reviewer but the editor who is managing the process. The communications in the reviewing system are thus the record that they have available to work with and that form the basis for their judgement not only of the paper but of the quality of each review and the weight that should be given to each reviewer's opinion. Moreover, if there are questions that arise later around the paper, that is what will be used to audit the process that was used to approve the paper.
If a reviewer takes their communications outside of these channels, then they are essentially choosing to hide their communications from the editor and the scientific record (even if it's a normally confidential portion of that record). That can have bad effects in both directions, causing a good paper to appear less supported (because they wrote a weak summary review based on a complex communication) or a bad paper to appear more supported (because information was communicated that weren't actually in the paper).
So what if a reviewer has already been communicating with the authors and then is asked to review? Well, first of all, they should let the editor know; the editor might or might not see this as a conflict of interest and wish to choose another reviewed. If the editor doesn't have an issue, then the reviewer should at that point stop having out-of-band communications and communicate about the paper through the review system---where they might be able to turn around a review extremely quickly if they already have a well-formed opinion of the paper!
But should any of this affect your choices as an author? I don't think so. Ultimately, the reviewers are just some of your first readers, and the impact of your article will be determined by the many more opportunities that people have to read it after that. If people are contacting you to talk about your work, that is already a great sign---even if they don't like it, they were interested enough to engage with it!