I finally found a technique that helped resolve the flush fit issue I've been having with my iStick 30 and 510 connection toppers.
Using a pair of needle nose, toothed type pliers (a Leatherman tool), combined with one of those flat rubber things from the kitchen that help you screw off tight jar tops (to protect from scratching either device), I was able to screw the Mini Nautilus down down with some brute hand force. Then I screwed it off again, and repeated, using a bit more WD40, until I got it down to where it is pretty much nearly flush on the iStick, with a very barely noticeable air gap which you have to look hard to see. I can now also screw the Nautilus off fairly easily, though it still takes a bit of extra effort when it is down tight (there is not a lot to hold onto for screwing the Nautilus in and out, as the airflow adjustment dial gets in the way). The ohm reading changes upward slightly after I did this, but not significantly from the slight variations I would have before anyway (+/- 0.2 ohms), depending on how tightly or loosely the Nautilus was screwed on. Currently, after a bit of vaping, the 1.6 ohm BVC head reads at 1.9 ohms It has never read as being 1.6 ohms on anything else I've tested it on, though it is marked as a 1.6 ohm head on the head itself. That is apparently not unusual for Aspire heads.