It's a bit of work to check even one track with Audacity, but the five examples I've shown at the top of this thread are the only five tracks I've checked - one track from each of five different albums, but I've downloaded many more 96/24 albums from HDTracks, and frankly, I'm not interested in checking the rest of them. I literally don't want to know about having been ripped off (whether due to malice or negligence), when it's too late to complain - especially having found problems in two out of five randomly selected tracks. I wouldn't feel prepared to file a complaint with HDTracks until I had expended a LOT of effort checking every last one of them.
What's done is done, but from now on, I'm going to check the first track of every album I download (from any provider of "Hi-Rez" music), as soon as that track completes and the 2nd track starts to come down. I will cancel the remainder of the download, if ever I find another 44.1-kHz file disguised as an 88.2- or 96-kHz file. I'll then contact the vendor, explain why I cancelled downloading the remainder of the album, and demand a refund.
Checking at the time of download makes a lot more sense than complaining months later, and in truth, I've not brought this to HDTracks' attention, nor given them an opportunity to correct the wrongdoing, because I'm not willing to spend countless hours checking every last track I've ever downloaded from their servers.
I intend to continue doing business with them (if they'll have me) because I really do like some of their remasterings, but I'm going to make sure that the butcher doesn't have his thumb on the scale.
Ignorance was bliss.