1. I've had several guests whose accounts showed they had been members for a couple of years, yet had no reviews. It all 3 cases, it turned out to be quite innocent- their past hosts hadn't bothered to leave a review, or they joined planning to travel but never had the chance, or their past stays were booked under their partner's account. In all cases, those guests communicated well with me and all turned out to be great guests.
There's nothing wrong with saying "Hi XX, I'm just curious as to why I see you've been a member for a couple years,. yet have no reviews as a guest."
2. Yes, all reviews go public regardless of whether both parties left a review.
3. Yes, you can ask a guest to please upload an actual face photo, if you have that listed as one of your requirements. But you do realize, I assume, that hosts can't see a guest's profile photo until after a booking is confirmed.