I had a solitary tomatillo last year. Dropping blossoms, no fruit. Then I found out about the cross-pollination thing.
Is there another tomatillo at the 'better location' ? I don't imagine it'll be worth your while moving the plant if there isn't.
Baking soda and diluted
milk are quite effective on the early stages of powdery mildew if sprayed on really thoroughly and repeatedly.
PM always defeats my cucurbits by season's end and I generally just let it go, but it's still pretty early over there isn't it?
Actually, are other plants showing symptoms? A brutal, yet practical option might be to cut your losses and chuck the lone tomatillo on the
compost if it's a PM carrier, with no likelihood of producing.