During my n-billionth attempt to beat Carn Dum, I came across this situation: Snowmane is attached to Theoden, and I've just quested for a net total of 1... except an Orc Grunts is in the staging area. He says:

"Any time progress is placed on the current quest, reduce the amount of progress placed by 1."

There is no active location, so I don't place any progress at all. Did this mean I have not quested successfully, as for Snowmane's trigger?

I think you quested successfully but didn't place progress. So Theoden would ready.

Ah, you're right, FAQ 1.24 defines it quite precisely: "As soon as the players determine that the total committed Willpower is greater than the total Threat in the staging area, they are considered to have quested successfully."