I agree with Dave, this is a misdeal as two cards were exposed. Also, I agree that the players who acted out-of-turn do not represent substantial action as their play was illegal.
According to RRoP v11: 15-Tournaments #13. A player must be at the table by the time all players have their complete starting hands in order to have a live hand for that deal.
Also, TDA Rule #30: At Your Seat and Live Hands
To have a live hand, players must be at their seats when the last card is dealt to all players on the initial deal.
The Button player did not have a complete starting hand. The initial deal was not complete as the dealer was still in the process of dealing the final card to the button. Therefore, none of the players had a live hand upon which they could legally act.
TDA Rule #35-D says ~ that a misdeal cannot be declared after substantial action (SA) occurs. That governs situations where the deal, otherwise thought to be complete, might be considered a misdeal had SA not occurred. For example, an entitled player not at the table, was not dealt a hand. The difference here is that the dealer and all other players at the table believed that the initial deal was complete. Subsequently, substantial action occurred before recognizing an absent player without a properly dealt hand. In this case, a misdeal cannot be declared and the hand would continue.