The question came up over on agotcards, and I sent it in to FFG (before I'd looked up this thread, incidentally). The answer just came in:
The Agenda "Defiance" says: Your plot deck must contain exactly 10 plot cards.
The plot "Betrayal at the Wall" says: Increase the size of your plot deck by 1.
If I run Defiance and BatW, can I increase my plot deck to 11 cards total? If not, why?
You can increase the size of the plot deck to 11 cards with Defiance and Betrayal at the Wall.
The core rules say, “Your plot deck must contain exactly 7 cards.” (Rule book, page 24.)
Defiance adjusts the above line, to “Your plot deck must contain exactly 10 cards.”
Betrayal at the Wall increases the size by 1 in each case.
I must say this is reasonable. Ktom's earlier answer was certainly reasonable too, seeing the "to a minimum of 10" on the Treaty Agendas as setting a precedence for "exactly 10 cards" on Defiance was certainly valid, absent an official ruling.
The "to a minimum of 10" on the Treaties sets a hard lower limit, in the same way "to a maximum of 2" on Blood-Crazed Screamer sets a hard upper limit. OTOH, "exactly 10" does not set a hard limit in any way, it just replaces the CS Rules' "exactly 7", and can thus be modified in the same way. I can get behind that.