Ok, I'll take a stab at this.
1.016 does seem a bit high. Not classic "stuck fermentation" high, though. Cider is typically lacking in nutrients (though this is not always the case) and fermentation can be slow. I typically leave my ciders for a month before doing anything with them.
It could be the yeast - yeast from a kit can be old or stressed - and could also depend on how much was pitched and if the juice was aerated prior to pitching. Though since I'm guessing you pitched a packet of yeast for each batch and had the same results across batches, it wasn't a bad yeast packet. Perhaps you've reached the attenuative properties of that yeast, but I doubt it.
It could be unfermentable sugars or other compounds in the juice, but I doubt it would be enough for a FG of 1.016.
My first thoughts:
- waiting a little longer won't hurt anything - I'd recommend waiting another couple weeks for sure
- you can try to swirl the vessels to rouse the yeast, and maybe move them to a more consistently warm location (though 18C is likely warm enough)
- check your hydrometer against some tap water - the paper thing can sometimes slide and give a bad reading. I heard a story of a commercial meadery that threw away their house yeast strain because of what turned out to be a bad hydrometer.
Failing that, you could:
- try a forced fermentation test to gauge whether the yeast is done, or just stuck
- pitch some fresh yeast, maybe from a starter (maybe yeast nutrient too, but I don't know what effect that will have mid-late fermentation, on the fermentation or the taste)
- maybe even pitch some more attenuative yeast, like a champagne yeast. You could even try this on just one batch of the four and make an experiment of it.
Hope some of this is useful - congratulations on 87L of harvest, I'm jealous!
EDIT: It also depends on what you want as a finished product. Mainly, you just don't want bottle bombs. But if you decide you like the taste a little sweet and don't want to carbonate, you could also stop the fermentation with sorbate, etc to be sure that it won't ferment any further. Or, you could stop the fermentation and keg.