Tea Drinker, I think you're right on both counts.
Some of you asked if DH was aware of the severity of Doug's condition. I think he only knew that Doug isn't fond of traveling. Just as an example, he knew that last year, Doug and his family flew overseas and had a great time, but it wasn't until after they discussed this trip that he found out the rest.
DH spoke to Doug this evening and confirmed that he will be joining him on the trip. Both of them are very excited.
As for some of the details of the trip, it's a non-issue. Other than his disability, Doug is a fully functioning adult with a job, family, and home, he's not prone to being clingy or dependent. When DH is off doing his company stuff, I'm sure Doug will enjoy himself.