I think the Vet would have mentioned Cushings if it had been likely. And it does tend to be large patches on the flanks. Also accompanied by skin issues and swollen belly sometimes - plus back leg weakness. So if it is just fur round the face I wouldn't worry about Cushings.
Is your vet an exotic vet? So the only real changes have been new substrate after a cage clean, and different sand. It could well be the change in sand as it is all her head area, and they tend to wash their faces and behind their ears with their paws quite a bit. How are her paws?
I think you'll need to go with process of elimination. If you've been using the Tiny Friends sand without issue then switch back to it. There is nothing wrong with it. It does look different/changed, but our robo was fine with it - our Syrian was slightly disgusted with it but it hasn't caused any harm.
Maybe try that first - it would be a bit stressful to change all the substrate again so soon. So maybe go back to the previous sand, and then if it hasn't improved in a week's time, change the substrate. Have you tried Fitch? It works out cheaper than Carefresh and is hypoallergenic - it's just recycled food grade paper. I get the 10kg bale which costs £22 (just gone up in price). It's not that massive to store and lasts for ages.
It is possible she could have developed an allergy to Carefresh even if she's used it for some time.
The increased drinking is something that happens with older age generally as their kidneys stop working as well. It is also a symptom of Cushings - but without the other symptoms I would think this is just normal ageing.
Our Syrian went like this around the face last summer - and it was just a moult in the hot weather so it could be all kinds of things.
This is the Fitch - if you're going to change all the substrate again in about a week it might be worth switching to this. It is sold in smaller bags but they're not big enough and it works out quite a bit more expensive that way as you'd need 3 or 4 bags!
https://www.fitchfirst.co.uk/fitch-p...-delivery.html
So process of elimination. Go back to the old sand, then change the substrate, then see how she is. If no improvement then back to the vet - and if your current vet isn't an exotic vet, maybe see if you can find one in your area for a more specialised opinion.
Did the vet do a skin scraping for mites? Because that is also a possibility. Mites naturally live on them as normal, and if their immune system becomes lowered, they can then have an outbreak of mites. They shouldn't really treat for mites without doing a skin scraping and looking under a microscope to check and give a clear diagnosis. If it was mites though it's likely she would be scratching a lot and making her skin sore with the scratching.