A lot of rejigging, but eventually I got...

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Huzzah!
The sprites now load like they oughta. The editor is editing, saving to the Symbol Data in the code, and when run, the symbol data is loaded and the sprites are functional.

It's not "massively" noticeable in this video, but every time I hit the Save icon (which currently isn't an icon, I need to draw some icons!) the Symbol Data for Symbol 0,"" changes, due to my edits.

I still have a number of elements I want to add to the editor, like drawing rectangles and circles, and a flood fill and such, and goodness knows I need an Undo button, but ... Yeah, that works!!