I think you are behaving quite rationally and so is he.
I think he is angry and doesn't know what to do with the anger. I think 5 year olds deal (generally) quite badly with disapointment and anger, and he almost certainly needed a transition period.
I think he is also being a PITA with it and your response, while not a Perfect Mother/Stepford Wife response, was normal and proportionate. I don't actually think that children are scarred for life by seeing their parents get angry with them when they are being eegits. Also, I think it is good for them to see what happens after one loses ones temper-you calm down, apologise, etc.
And you did all the right things with the understanding and so on. Sometimes they are just angry. Sometimes they just need to be angry and then, IMO, its about recognising that they are deeply pissed off and letting them express this, but with boundries to protect others. We can't stop them being angry, and nor should we.
I think there is a difference too between sending a child to their room to get some space between you-bascially to prevent the situation escalating-and doing it as a behaviour management tool. I don't really hold with the latter but the former-well its unavoidable with some chidlren, isn't it?
I can see that some anger management techniques MIGHT be helpful but really-he is 5. He will probably grow out of it.