I'm not sure what Ernst is speaking of with his paper sandwich either, but am intrigued as well. My only suggestion is to avoid any mayonnaise on the sandwich, as I've found the printer fairly intolerant of it.
As for your question about how to load the target, it's been a while since I did it, but remember wondering the correct order, as well. I loaded the profile print like it was normal blank media, then went to APS for the next steps.
I'm curious if this works out for you, so please post your take on using the coated profile when you are done. Ernst brings up an interesting point about the calibration being different and not being coated, but it seems like that keeps the printer in line to print the canvas the same each time, and the profile should be able to factor in the coating as a separate adjustment if you build a profile with coated canvas. I've heard others mention this approach for external profiling, but don't recall anybody mentioning this with HP and APS profiling.