No, this site will usually accept your bet pretty quickly, with 0 confirmations. So, if you're watching a match, you could definitely hit directbet pretty big using this tactic. Once your bet is accepted, they cannot cancel it. Just like they didn't refund mine even though my player got hurt.
I am certain it would not be accepted. In the future if you place a bet which is received by the bet id address after a player retires(in the case your player retires and loses) , and Directbet quickly confirms your bet and results it in a loss after that , then this really wouldn't be right. Even a single case of someone having proof of this could result in the loss of their reputation.