What should I do with the airflow in a sealed off conditioned crawlspace
Sounds a lot like my son's new house.

Since you already have an HVAC duct that runs through the crawl space just add a register to that duct. My guess is that unless you're actually going to seal the crawl space and make it air tight relative to the rest of the basement, you'll have enough leakage around the access door so that no additional return is needed.

I don't think the dehumidifier is needed with the air from the HVAC system.  In my son's house, the crawlspace is open to the rest of the house.  To be honest, it's a bit more than just a crawlspace in that the height is ~5 ft, but still with a concrete floor and insulated and sealed walls.