Flying on Southwest with a cat

My wife will be flying Southwest in a few weeks for our move, and will be taking our cat (while I drive cross country - we figure flying is better than driving for 4 days!). We're looking to verify what's allowed carrier-wise.

Southwest has the following page on their pet policy:

• The Southwest Airlines Pet Carrier (17” long x 9.5” high x 10” wide).
• Other pet carriers with maximum dimensions of 18.5” long x 8.5” high x 13.5” wide.
• Soft-sided and hard-sided carriers specifically designed as pet carriers are acceptable.
• The carriers must be leak-proof and well ventilated.
• The pet carrier must be small enough to fit under the seat in front of the Customer and be stowed in accordance with Federal Aviation Administration (FAA) regulations.

8.5" high is very low - our cat is nearly 8" tall, and he's a pretty small cat all things considered. We've seen lots of reviews of carriers that are a bit taller that suggest they've been successfully carried on Southwest - like this one. Obviously the underseat space is only 8.5" or so, but a soft sided carrier would easily deform to fit that - while allowing the cat room to move around more comfortably during flight while on her lap (in the carrier, our cat is definitely not an outside-of-the-carrier cat). Southwest's official carrier (which is sold out) is itself 9", not 8.5"...

Is a slightly taller carrier likely to be allowed to board? (We're considering getting two, I'll be at the airport anyway in the car in case of a problem, and I could keep the smaller one.) I assume if we ask an agent, they'll say "no" since it's not explicitly permitted by the rules - but our concern is mostly that she gets all the way to the boarding stage and then is denied boarding (at which point there's not time to come back and get the carrier) or else is denied by the flight attendant. Is that a significant risk, even if she does ask a gate attendant?

Alternatively - has anyone flown with a smaller (9" or so) carrier with a cat that took up most of the space vertically? Are we overworried about the space issue? He will likely be partially sedated (per discussions with our vet), so perhaps it won't matter too much...