I don't know personally what robbing looks like, but I wanted to comment on the other stuff.
I'm thinking you should open up the entrance wider. They should be strong enough to not need the reduced entrance now. Also, the hive is sort of like an airport. I've seen many times there gets to be a "back-up" of incoming flights. With you're small entrance the back up would be worse.
Here's approximately what a bee has to go through to get in the hive:
- first check by guard "do you belong to this hive?"
- second check "did you bring what I asked?" (they are told to bring pollen or nectar)
- transfer of goods to another bee
- possibly told where to go and what to get before take off
It's a slow process, and slowed down even more if the entrance is small. Just like at a busy airport, if you only have one line to go through (instead of having 10 lines available), it gets pretty backed up and takes alot longer.
And a word about making a split. Not sure if you want to try that this late in the year. It's best to go into winter with two brood boxes, especially since you do have snow in your area. I don't get snow where I am, and I don't even think I'd want to try and do winter on only one brood box. My first hive was started from a 2 pound package about the 4th of July. The just barely made two boxes, filling 7 or 8 frames with foundation (and stuff) in each box. So if you can get a queen quick, then it might be worth a shot. But if not, then I wouldn't think it would be wise to try and get the bees to raise a queen from a split. That would cost them alot of valuble time. And it's even more doubtful that both hives would have enough honey to make it through winter. You'd have to feed them. Not really trying to discourage you from the idea, just giving you something to think about.