Some things need to be cleared up here.
1:
Lehmann had to go. Period. End of discussion. If it had been Rovers vs. Gunners in a cup final & Bellars had been the victim the reaction here would have been hysterical; using both meanings of the word.
2:
It IS within the referee's discretion in this case to allow the play to continue, count the goal & then pull out the red card (which is what should have happened). Last man does not even have to come into the equation. The goalkeeper pulled a player down, whether intentional or not, outside the 18 yard box. That's a red, and that's that. The ref then allows the advantage to be played.
For those of you who were more interested in a night's entertainment than seeing that the deserving team won: "Go watch clowns."