I am not going to be much help, but understand where you are coming from. I bought 9 RNTs 6mths ago and still have all 9, all looking fabulously happy from the get-go despite a 5 hour drive home and then a bumpy 6km driveway.
BUT, I have lost every Cory or Otto I have put in there with them (except those I rescued to other tanks once I realised there was a problem). So sort of the opposite problem to yours.
I now have 2 Ladder Loaches in there (testing the waters so to speak) and now that they have been happy for a month I am thinking of more. I suspect my (large, local) driftwood of releasing something, but some stuff just remains a mystery I guess.
Mine were bigger than yours I think, maybe 3-3.5cm. And evidently there are different species of RNT, some of which might be hardier?