I actually really liked the 1x1 mode and found it very helpful. I first started using Reorder with 1x1 as part of a memory experiment that turned out to be highly successful: always review the reading-then-meaning back to back, because doing the reading first helps you think in Japanese faster (search “reading-then-meaning” in the forums if you want the reasoning).
Of course, you can still do reading-then-meaning without 1x1 by just reciting it that way in your head every time, regardless if it’s asking for meaning or reading. That has also proven successful, so 1x1 isn’t strictly necessary.
But the other significant impact for me was that it made things far easier for me to remember. The brain loves consistency and sequences, and especially consistent sequences. As long as I repeated the items over and over during the initial lesson until I could remember them without hesitation, then the back-to-back helped cement the reading and meaning as a pair in my head, and reviews were much smoother and easier later. (But, if this doesn’t work for you, then it doesn’t, so do whatever works for you).
As for interleaving, the principle actually refers to bringing things together that would normally be separated by a day or more. It’s not about mixing things up for the sake of separating them. The reasoning is that doing related things together helps you discern relationships that you wouldn’t have noticed if you had done them separately. So, it really doesn’t apply to 1x1, or even to readings and meanings in general if they were already going to be in the same review session.
But, as always, people are different, so just try various things, and stick with what works best for you. 