OK, so if I am staying in WDW the night before our cruise and having DCL Pick us up, do I have any control whatsoever over our boarding time? Does DCL assign it for us? They asked me to choose a boarding time when I pre-registered a few days ago but I have zero idea if the DCL bus can get us there early enough to make the time I selected.
Is this something I need to call them about? I'd hate to find out they can't get me there by our boarding time and find out I messed something up.
I think you may be confusing boarding time and arrival time.
You don't pick a boarding time, you pick a port arrival time. You pick one even when on Disney transfers which is funny, because as you said, you have no control over your arrival time.
Disney started asking guests to pick arrival times a few years ago to try to spread out arrivals because the terminal fills up quickly when the Dream and Fantasy are sailing. No matter what time your arrival time is, unless the terminal is full, you will be allowed in. If it is full, you will be held outside until boarding begins and the terminal clears out a little.
The arrival time you pick will play a part in your boarding number. The earlier the time you pick, the lower your boarding number. With a 10:30 or 10:45 arrival time, you'll have a very low boarding number. Around 11:30 (sometimes earlier) they will start calling boarding numbers. By 12:30 or so, all boarding numbers have usually been called and it's in open boarding, meaning numbers don't matter, and everyone can board as soon as they check in.
Because you are on transfers from WDW, you will probably not arrive until open boarding has begun, so your boarding number won't really matter. If you are lucky enough to be on an earlier bus, you'll probably still get there after your boarding number has been called.
So check in and enjoy walking on without waiting!