Yes.

Whatever definition of historian you choose to use, from having published books, holding undergraduate or postgraduate degrees, employment at universities or museums or whatever else, we have flaired users who will meet the criteria. From our perspective though, we don't judge user contributions based on their qualifications or professional credentials - we have no way of systemically reviewing such claims. Instead, user contributions are evaluated on their merits, and flair awarded based on a strong, sustained track record of providing high-quality answers to questions on the sub, demonstrating their knowledge of sources and scholarship in their claimed area of expertise. By this standard - the only one that is practical to enforce on an anonymous platform - hundreds of historians participate on the subreddit.

Yes there are. I'm assuming that you are wondering why more questions aren't being answered. TBH, most questions are too broad (i.e. what was it like to grow up in the 1680s), too obscure (i.e. what was basic training like for Sargon of Akkad's spearmen?), or sadly, kinda dumb. Then, if the question is interesting enough, and able to be answered; still someone must feel compelled to post a subreddit compliant answer. That is, knowledgable. This takes time. Requests do not incur obligations

There are quite a lot of them.