Update: While adding KEYFORMAT="indentity" to the #EXT-X-KEY tag in the m3u8 file did prevent the aforementioned crash on Simulator, strangely, this tag edit now prevents the AVAssetResourceLoaderDelegate from getting called on the device.

Update 2: Revisiting this for 15.2 and adding the KEYFORMAT="identity" no longer prevents crash on Simulator. Maybe it was just a fluke that it worked before...? Anyway, HLS w/custom url scheme still works on device, just not on Xcode Simulator.