I believe wing nuts can be safe and secure and removable using no tools, BUT the threads and the nut/washer interfaces MUST be kept well-oiled.
Just as important might be that the hub's driveside axle locknut present a sharp and well-formed knurled biting surface to the dropout. Some locknuts are inadequate!
I've never had a problem with wingnuts and I've done quite-spirited rides here in the foothills with periodic 20% inclines.
As for a 15mm wrench, I use a 4" adjustable wrench, some examples of which I had to file away a half-millimeter to reach 15mm. But one of them also serves to remove my Varsity's rear nuts which are fully 5/8" or 16mm across. Again as with the wing nuts,
threads and washer interfaces kept well-lubricated with ordinary oil.
My customized MTB uses hex nuts front and rear, and my 4" adjustable wrench is all I've ever used out on the trail:
Camera parallax can be a weird thing, this wingnut actually turns without removing the rear derailer:
