Yes, I've found it virtually impossible to set a full-spectrum Custom WB on my SD10 or SD14 but the SD1M will take it most of the time. So, I use Incandescent on the earlier cameras.
A thought occurs that the so-called 'UniWB' might be worth a try:
GUILLERMO LUIJK >> TUTORIALS >> UNIWB. MAKE CAMERA DISPLAY RELIABLE
I did it on an SD10 long ago, and it does involve shooting a wine-colored target on the monitor (but with the dust cover in place). As you may know, UniWB attempts to make the camera histogram show raw levels.
From your post, I've just realized that Custom WB does affect the camera LCD and histogram, so there's an area for further experiment in order to help with assessing what they are telling you, which is mostly lies once you've got the 720nm on the lens