They have probable cause and would most likely not let you re-enter the apartment and if you tried you would be arrested for hindering a law enforcement investigation.
I agree with what you said but I don't think it would have mattered. He shouldn't have even opened the door.
Not in my understanding. If they don't give consent, those officers cannot enter without a warrant unless they have probable cause to believe that somebody in the home is in danger, or if they've already made an arrest and they believe somebody inside the home is destroying evidence related to that arrest. Probable cause based on simple possession and even possession with intent to distribute will in the vast majority of cases not be enough for them to obtain a warrant after the fact, unless the officer established "plain view" of the offending material.
From the description of the scenario they smelled it and then asked and were granted consent for a search, which was dumb. If the consent is not granted, the officers have no further recourse to perform a search of the house based on the smell alone.