I can think of 3 reasons you're getting blowback on the DE test.
First, the obvious: It's your multivalve.
Second, you may not have sufficient sand and it's passing through
Third, It's also possible your laterals are cracked or broken...but that explains the DE, not the leaking.

You could fix or replace just the multivalve. If the problem goes away your are done. If not, you'll need to empty the sand and inspect/replace the laterals, and refill with new sand.
It should STILL be cheaper than replacing the filter.

Of course, you could replace the whole thing with a bigger filter. I'm wondering if the pump is too big for it....what model and brand pump is it?

Carl

Carl