Hi, and welcome to AFF!
First question: We have no problem with you crossposting your work to multiple sites, including AO3. Many of our members post elsewhere as well as on AFF.
Second question: If you’re not using the same pen name on other sites as you do here, a quick author’s note at the top of the first chapter, to let readers know you are the same person as your AO3 persona, is generally enough. If someone comes forward at some point and claims to be your other pen name, that would be an issue, but we do have members who use a unique name here to write under, because we host adult content.
Third question: We do ask that tags be used for even a single instance of most things, but we don’t require that certain tags like WIP or Completed be used. It’s been our experience that readers can find any number of things upsetting if they encounter them unwarned, and that’s the purpose of tagging something. We’re particularly vigilant about tags which are very likely to be disturbing: Rape, Minor1/Minor2, Incest, Abuse, et cetera. Those really are trigger issues and we never want readers to walk blindly into that content. If a tag is missing that we feel is required, we do contact you and even if the story is hidden until the tag is added, we don’t summarily delete stories for a missing tag until you’ve had at least 40 days to make the correction from the date of the initial warning. We’re actively moderated, and we aim to response within 24 hours to any inquiry, or email about a correction.
Fell free to ask any questions you have—we’re happy to answer!