I found out the root cause!
I discovered that there is some kind of conflict or anomaly between SeaTools and HD Sentinel and this model of Seagate drive. I noticed these errors did not generate in HD Sentinel unless I started the Basic Tests in SeaTools. The Short DST test in particular is weird because as soon as I start the test it triggers the Hard Disk Test in HD Sentinel automatically (although there is about a 20 sec delay for the GUI to update), and two errors were generated in HD Sentinel. When I cancel that Hard Drive Test in HD Sentinel, it causes the SeaTools test to fail (when left alone, the SeaTools test passes). If I abort the SeaTools test, the HD Sentinel test automatically stops about 20 secs later. I was able to duplicate these events 5 times. I tested this scenario on an HGST drive and I could not duplicate it, so it seems to be a brand or model anomaly.
Regarding the errors in the HD Sentinel log, while they don't appear every single time a SeaTools test is run, every time they do, the time matches up exactly with when I started a test.
I've attached a screenshot of a couple of errors triggered by SeaTools (notice the HD Sentinel test, which was triggered by SeaTools). Also attached are my results from a Read Write Read test which seems to further prove that the drive is stable and that the data/power connection is solid...or, at least I assume it proves that.
Thanks for your wonderful program and all your feedback...I learned a lot from this experience!