I suppose if I was not ever going to use the 18T supplied with the bike a link might be able to be removed. Would the difference between 18T and 15T allow that I wonder?
A quick Google search reveals that the 2015 Flyer
reviewed here has track ends (rear facing & not drop-outs). That means brakes get closer to the tyre as the rear axle moves back. So a smaller sprocket (or chainring) may cause brake problems, as you found.
Shortening the chain is a pragmatic solution. Removing a whole link (i.e. one inch) would match a reduction from 18T to 14T (not exact, but close enough). I suggest inverting the bike, loosening the rear nuts/bolts/QR & seeing whether you can move the axle far enough forward to fold the chain a whole link shorter.
There is more than one way of shortening a chain. You could use a half link (i.e. half an inch) if a whole link was too much. Joining links (re-usable ones like SRAM & KMC) allow you to take a bit of chain out & replace if you need to.
Chainring: bolts sound worth investigating, it's definitely just the one 'hotspot'. In fact, I checked the bolts to make sure they were properly nipped up and was very surprised to find that they were not. I suppose they must have worked loose after the bike had been assembled and then ridden for a bit. I only used a multi-tool, so not much torque was used.
As ever,
Sheldon Brown offers good advice
Slack: I was going from the middle of the top chain run (middle being between the cog and chain ring). Sounds like it is taut enough, but thanks for the derail tip.
My approach (not original) is to tighten as far as possible without the chain binding. That usually means overdoing it & then sliding the axle forwards just enough that the the chain doesn't bind at the "high spot" of the chainring.
I re-tension when the chain slack is enough that I can feel it when going from propulsion to leg-braking at low cadence (Andy Gates, formerly OTP called it flibble)
I think I am much more safety concerned with this bike than others I ride, because I feel right on the edge of control when riding it fast down hills.
Agreed. It takes time/practice to build confidence in your ability to control fast descents. A good back brake is important if you feel you're losing the battle. It's a learning process. You need to find your own way of coping with legs whirling at a silly rate.