Hello Paul,
I run WDT P328 24x12x12 tires, with the rims flipped around to give a wider track. I'm fairly sure I ran them in the narrower track orientation for a while when I first mounted the tires, so the width should work either way.
I'm less certain that the 26" diameter would clear the downturn of the fender...you could always roll the edge of the fender up, which would weaken it, not that it really needs to be strong. That said I don't see much advantage running 26s over 24s.
Regards,
Ben