Way too much work in a free form to notch or cut skinny pieces. What you do is use release paper like tyvek housewrap and lay that in the void carefully and mud over the last 2" so its supported but can slide with the deck. Wouldnt hurt to put some fiberglass mesh or metal lathe strips in that section. We use fiberglass mesh that's intended for repairing plaster walls it's on big rolls I think homedepot may possibly carry it.
This is one of those situations where like you said 14" coping would have been ideal. You learn alot building a pool first time the 2nd goes way faster lol. By the time you do a dozen or so you get the foresight down pat
I guess it would all depend on how confident you are in the backfill process. The area in discussion is the most likely to settle if there was improper compaction in that area. So instead of the typical 1/8 to 1/2" settling lip you typically get you could end up with a more unsightly joint with cracked pavers. Typically gunite pools only have a few deep fills (ramps or plumbing trenches ect..).
Agree with the poster about the Tyvek, although it might be better to have something a tad thicker with some "air" in it. You just need a de-coupler to separate any movement from the decking from the shell. Thin foam would work in a pinch if most of the pieces cantilevered the gap. In theory, you could pull the 1/2" foam out later and put in a nice caulk to facilitate movement.