OK, I'm not too proud to admit making mistakes, myself.
I've just double-checked my new 64gb MicroSDXC card, and it seems I unwittingly dragged and dropped some folders onto it that I hadn't intended to. Consequently, although I had no reason to expect it, it turns out I accidentally had some duplication of a few folders from my 32gb card onto my new 64gb, hence my apparent finding that the DX100 was showing files that (I had every reason to believe) weren't on the new card. My mistake and I hold my hands up to it and admit it. Sorry iBasso The other 3 points still stand, however.
I've also noticed that haptic feedback still vibrates the DX100 when I switch it off, even though I disabled the 'haptic feedback' option in the relevant Android menu.