CONGRATS! Sounds like it went really well!! Especially the getting cornered part.

You both held your own; that's great!!
I'm going to ask a stupid question here, so don't be offended and think, "Are you kidding?
Really, she's asking me
that???"
How did you start with leaving him with the trainer? Since the issue is him
watching you walk away, maybe you should work a little more on
distance, not just
time. In other words, kind of like with stay, put him with the trainer or whoever, but just walk maybe a step or two away. He gets to see you walk away, but you come back a lot sooner than he's used to, and you don't go as far. Basically, expand his comfort zone until he's comfortable with you walking further away. More distance pretty much equals more time, because it takes you longer to walk further away, so you're really working on both, just with a little more emphasis on distance. If he's really not happy when you even go a couple steps away, then you may have to take super baby steps. For instance, hand leash to trainer, turn your back, and then turn around and reward. Then turn your back, step forward, turn around, reward.
So did you start with distance, or did you just work on the time that you are away?
Like I said, if I'm telling you something you already know, just disregard.
Glad it went so well!!