I actually did figure out the trick about editing the G code. I'm rather shocked that I had to go under the hood in that way for a setting that seems so basic and so obvious. I'm surprised it isn't in the quickie panel that first appears when I go to generate gcode, right underneath the print temperature setting. Why not??

I haven't tried spraying anything yet, I may do that. Based on someone else's posted advice, I have tried printing with a heated bed temperature of 70 – 75° C, and this actually helps considerably (though not perfectly) with the curling corner problem on large flat objects. I'm avoiding raft, it doesn't come off the printed object cleanly at all!