One other thing I just thought of..... When we got the dust mite diagnosis, the vet recommended we get a bed for Dexter that was machine washable on a high temperature and wash it once a week. So I do that now at 60 degrees, but I also cover his bed in a blanket that I change every other day.
We bought a new vacuum too, but we needed one anyway so that pushed us to do it. I also try and vacuum every day. He very rarely gets upstairs into our bedroom, as I know beds and under beds can be really problematic for any dust allergies.
Oh - something else too - any plug in air fresheners we were advised to get rid of as they can agitate too apparently.
I hope you can find something that stops their eyes watering. It's frustrating when you just can't find the cause of it.....