utterly, I am not an expert educationally but it does sound to me as if he is very bright actually. I think self consciousness can go along with a bright and sensitive child.
Maybe his personality somehow 'clashes' with his position as number 3 - as you say he has to fit in and spend time in the car etc and is at the bottom of the heirarchy. Some of his behaviour may be prompted by a part of him that is rebelling against this - as a first child he may have been easier! I know this is not helping as there is nothing you can do about it, I just think understanding more of his motivation may just let bits of positivity toward him creep back in - if you feel he can't 'help' it in a way....
I am glad you liked the parenting course idea, I do hope you find something that works.
Your DS has struck a chord with me, my DS is also at his happiest one on one with adults, is self conscious, loves new places and is in bed exhausted at the same time as your DS! He can also be very attention seeking.
I think the main thing for now to keep your sanity maybe, while you look for some other input, is to keep to perhaps time out as a consequence. I really believe that if you are consistent and it is set in stone that he will get a consequence for certain behaviours, he WILL learn - even if he laughs or calls names at the time, he will be learning. But time out is only a punishment if you can give him some one to one attention as well, otherwise he is not 'losing' anything by going into time out. At least it's a break of a few minutes for you to gather your thoughts, as well.
I also think maybe you and your DH could sit down and see if anything can be done with your daily routines to make life easier. Is your DH home at dinner and bedtimes or is it all you?
You say homework time is a sabotage target for him - can you just for the moment move his bedtime half an hour earlier so that the kids can get their homework done then? I know this isn't ideal but it's a way of 'firefighting' for now to give you all a break!
Hope some of this is helpful, I really feel for you.