I would call to do the ST mileage upgrade immediately after booking, so that in case it doesn't work out you can still cancel w/in the 24 hour window.
The SkyTeam mileage upgrades are very rarely used, so most DL agents probably have little to no experience with that, so the one thing that you are guaranteed is inconsistency and uncertainty on being able to do it, regardless of if it is actually available or no.
I unfortunately don't know into what fare class it books, though it being a mirror of O availability would certainly make sense (though that's not to say that is how it is).