I think you did a great job - frustrating, I know.
We use a "naughty mat", so we can move it AND take it with us in the car - or use a blanket or towel as a mat. That way the bed or his room doesn't become a place of punishment. His behavior is associated with a moveable object...since I want him to enjoy his bed. You could have one in the car, make him sit on it and then return to the restaurant. Our son use to scream because he WANTED to leave....but I couldn't just allow him to scream, so he got what he wanted...but with the mat in the car, it was immediate and at that age, it worked well....and we returned to the restaurants.
Here's what I told another mom:
We have a willfull 21 month old that we have been sending to the "naughty mat" since he was 12 months old. It's the ONLY thing he responds to.
This is how it works: We give him a warning...they understand at 12 months, regardless of what people tell you. (The sooner you start to parent, the easier it will be.) Then, if the behavior continues or happens again, we put him on the naughty mat. (You can't just threaten, you have to be consistent, or nothing will work.) We move hom to another room, onto the mat - he usually finishes his tantrum without us watching, but we are still listening. We give him another 10-30 seconds (depending on his age) of him being silent and then we go over, have him stand, explain briefly what he did wrong, have him give us a hug and we tell him that we love him and we really enjoy him behaving.
At first, we had to put him back on the mat sometimes 3-20 times, as he would crawl or walk off - but with the consistency - it's WORKING! He finally getting much easier, knowing that he can't push us.
We bought a carpet sample for a few bucks and we can move it anywhere. We also used this for our first son, but he didn't need it until he was about 2.5 years old. We were on vacation in a hotel and he was misbehaving. I asked him if he wanted to go to the naughty mat? He replied with, "You don't have it here." I went into the bathroom, grabbed a hand towel and put in on the floor....then I asked him again. He was perfect the rest of the trip. Our 2nd son isn't so compliant....but it still works like a charm.
We aren't into negative reinforcement. We do TONS of positive, but for this age and this child...this is the only thing that works for most of his behavior modification.
PS: We taught both of sons sign language, but the first one only used "more" on a consistent basis. The first one, had 30 signs down. Different kids...like night and day.