We just noticed you added re-ordering to the MultiValueFieldDefinition. It's great to have! We had already implemented it on our own, so we are going to eliminate our subclass and use yours. The only thing we did that you didn't was the following:

When you use keyboard navigation to tab over to one of the move up or move down links, and activate the link, it loses focus. So if I want to move something up three spaces, now I need to tab all the way down to the link two more times. Instead we should just maintain focus on the same link so you can activate the link 3 times and you're done. Hopefully that's a clear explanation.

I'm including a really simple patch against today's trunk to illustrate. Untested but it's dead simple and really close to what we did in our implementation.