Long post of suggestions/ideas:
The drive has reached a max of 49 degrees Celsius over its lifetime, possibly in your case. This isn't amazingly hot, but could be cooler. Allegedly, heat can affect the fly height, and it's possible that continuous writing to areas close to one another allowed for increased heat outside of what the drives actually like.
Outside of that, only the high fly writes look even remotely curious. All other error indicators seem to say that the drive can perform normally.
I would try heavy testing with one drive activated/powered up at a time (or maybe 2 at a time), with plenty of breathing room. If you still get an increase in HFW but everything else looks good, I would keep the drives but keep an eye on other parameters from time to time. I would also make note of any logging errors that the host operating system might issue regarding a true I/O problem.
And of course if anything changes and you get no additional high-fly writes, something in your setup needs to be tweaked.
You might also try to run one or two of the drives in a vertical orientation, or possibly even upside-down, to see if the drive acts any differently. Plus see if many small files results in a significant increase in the HFW count. (Maybe this would force many seeks, allowing the read/write heads to be in proper position each time?)
Either way, the drives should ideally be fine. If the high fly writes--self-corrected by the drive--concern you, then maybe seek Seagate drives from a previous generation until the kinks of this generation are worked out. Or seek drives from Western Digital or Hitachi/HGST (I don't know how much technology is shared between the two). Maybe a comparable Samsung would perform without whatever causes the increasing count, though again, I'm not sure how much technology is shared between Seagate and Samsung at this point.