I thought ALL behaviour could be trained?
It can.....but even once you find the training technique that works for your dog, and the specific behaviour you want to train, it can take months of work to achieve the desired result
I'm sorry that isn't a particularly positive answer, but it sounds like you might have reached a point where you have to make some hard decisions for the healthy and happiness of the whole family.
I understand your reluctance to have Bentley in the bedroom - my OH is the same. But, if the alternative is continuing months of disturbed sleep while you try to find a solution, or even rehoming him if you are unable to cope; which would you prefer?
Perhaps you could use a crate upstairs - there are a range of soft, pop-up style crates on the market that you could use if space is an issue; and if he only goes upstairs to bed, and doesn't have access at the rest of the day, then hairs can be kept to a minimum
Dogs don't always fit our expectations and "house rules" - and sometimes, compromise is necessary in order for everyone to live harmoniously
Oh, and try not to worry about dog sitting arrangements; dogs don't generalise well, so sometimes, a dog which is used to being crated at night downstairs will howl the place down in a different house, and a dog which is used to sleeping in their owners room will settle downstairs on someone elses sofa with no problems !