Hi!
Answer 1: there isn’t a way to see if anyone else is connected to the box unless he/she gets the user/root flag and submits that flag, then it will be posted on the Shoutbox.
Answer 2: there isn’t a way to see if the box is 100% reset or still under influence by exploits etc. In a perfect world, everyone would reset the box after he/she completed it.
Answer 3: as far as I know there isn’t a limit on how many resets you do. Once you invoke a reset, your request for a reset is posted in the Shoutbox. There is a 2 minute grace period in which users who are busy completing the box can cancel your request reset. When no-one in your server group cancelled your request, then the box will be resetted after those 2 minutes. Note: a reset may take up to 1 minute (as i have experienced) until the complete box is really 100% reset.
Link to shoutbox: Login :: Hack The Box :: Penetration Testing Labs
Hope this helps