Anything that becomes molten will mix into the fuel and dilute it,
Not really. Anything that becomes molten, will pretty much vaporize, because Uranium melts at like 2000 F. If the Uranium is molten, everything else will boil away.
However: It's bollocks because the hole in which the uranium is burning, has fissures and crevases, and the Uranium would unevenly flow into small, tight spaces, spreading out and; ultimately diluting and cooling.
Experiments done at Argonne labs back a few years ago also suggested that the Uranium will form a cooler coating, as an outer shell. The core may remain molten, but the shell is cool enough to harden, and contain the molten core. The core may burn through the shell, but much of the mass will be left behind, as the molten part runs down into the burned-out cavity below, and the process repeats.
In any case, either of these scenarios would generate significant ongoing outgassing, and none of that has been observed at Fukushima; so it's likely the fuel melted and diffused and cooled. Just like Chernobyl.