I see no reason that Oxine would not be safe (in the right concentrations and in its UNACTIVATED form) for human consumption. England has been using it instead of bleach, in their public water supply, for a long, long time. I often feel the benefits of Oxine when I fog the barn, especially if I have a cold. I wouldn't hesitate in the least bit, to put some in a humidifier in my house. Again, in the unactivated form.
I have spoken with the US sales manager for Oxine and he doesn't feel I have to remove my birds when I fog/spray the barn with activated Oxine. I haven't received my citric acid packets yet, so I have not started spraying with the activated form, but when I do, I plan to use it just like I use the unactivated form. (I'll just be a little careful when I mix the citric acid into the Oxine and do this part outdoors - making sure the wind isn't blowing towards me.)
Regarding purchasing it, I have only found it online - and have not had any luck finding it locally in any chains. Maybe some day, as it gains in popularity, will it be available in feed stores or the like. At least I don't have to get it from Europe any longer!
Also, I don't see any harmful incompatibilities with using Oxine when you are also using diatomaceous earth. The only thing it could possibly do is cause the DE to be less effective, but there shouldn't be any problems per se, with using them together. KJ Theodore at Shagbark Bantams would probably have a better answer to this since she is a chemist and I am just a pharmacist.