[GhostsInSnow]

Hi there, firstly welcome to the forum. I just want to say I am more than sure you will be an amazing hamster mum on your own, it's clear how much you love your little ones and that really does go a long way.

In terms of Fili, what tests and treatments have the vets done so far? Have they done any little skin scrapes or anything? It can be a little costly depending on your vet but could give some answers if it's not already been done and it's not very invasive nor does it require anaesthetic.
When you've done mite treatments are you cleaning, disinfecting and treating his cage and toys too? Mites can live on these things easily.
If your other hamsters aren't showing symptoms then I'd imagine it's not anything that's come from the bedding.

Obviously I'm not a vet but have you tried ruling out allergies as that's one of the other big things that can cause fur loss. It could be a substrate/bedding allergy or an allergy to something in his food and this could be done by slowly ruling different things out. It might be worth mentioning to your vet to see if they can offer any guidance on the best way to do this.

I do hope you're little one will be okay