Bill, congrats on your progress so far!
You are right about the shaft collar omission from the parts list, sorry about that!
We'll have that updated in the next hour or so. The parts list has been updated in the downloads section.
Here's the link to the McMaster catalog page, its part number 9414T6 and there are three used in the current version. They're about $0.60 each.
The purpose of the coupler is to sandwich in the imperial bearing opposite the motor and keep it from falling out of the bracket. If you're getting a tight fit already, you can probably run without a coupler, at least temporarily.
You can also get by just fine with a piece of tape or a rubber band in place of the shaft couplers. We ran our test builds for quite some time with just a little piece of tape on the ends of each shaft, and that was enough to keep everything in place.
As for the belts, three equal sections (~4' each) will give you plenty of belt to work with.