Excellent that he is grooming
. Cushings is an over-production of the stress hormone, cortisol. There are two types - it can either be the pituitary gland that is giving out the increased cortisol, or the adrenal glands. The Vetoryl Coco mentioned works well for one type but not the other type (can't remember which though).
So the blood test should test for cortisol levels and if they are raised above a certain level that gives a clear diagnosis. I've linked an article about it written by an exotic vet if it helps. It does sound like it may be Cushings, but it can be so hard to tell without the cortisol levels being known, as ageing hamsters also drink a lot more and pee a lot more, as one of the first things to start wearing out is their kidneys.
Maybe it was cortisol levels the vet was referring to? If she was actually referring to liver function than that could be again just general old age. The liver detoxifies and if it isn't functioning well then it's harder to detoxify things which can lead to unwellness. It could also explain why drugs affect him if he isn't detoxifying them well.
Are you in the US. You mentioned not being to keen on your vet earlier. Is it an exotic vet? Because if not, try and find an exotic vet in your area - they are experienced in hamsters. General vets often really don't know much about hamsters and generalise a lot.
My last vet wasn't an exotic vet and I won't be using them again! I found it incredibly stressful not knowing if it was Cushings or not and the vet said it wasn't possible to do a blood test (which it is but clearly they didn't feel able to do it or weren't knowledgeable enough). For me it was important to have a clear diagnosis so I knew whether I could give him Metacam or not as he was a bit arthritic as well. But he didn't seem in pain or to be suffering until near the end.
I hope he picks up - he sounds like a grand little hamster.
https://supremepetfoods.com/molly-va...e-in-hamsters/