* You can book via your TA or CX.
* If your TA does not know how to do it, ask your TA to place you on waitlist and call MPC yourself.
* If M is closed for waitlist and the published fare is available, and Y is still accepting, your TA can book/waitlist you on Y. You can call MPC to do a seat guarantee on M. Note that only the booking class can be changed, otherwise your TA (even CX res) cannot have access to your booking. You will still be ticketing from your booking agent.
* There should be fares down to M, and sometimes to L or rarely V. It is highly unlikely that only YB or YBH fares are available. You can do seat guarantee down to V as long as fare is available. Note that fares may not be available for certain dates or certain flights. Sometimes fares are available if you ticket before a certain date. These restrictions still apply. If you need to book by 16 Jan for a flight by end of Mar, that fare is not available by 17 Jan even for seat guarantee.
* Fares can be very different for the same M bookings for different dates.
* You can still do seat guarantee when you need to change a flight after you start your trip.
* As a courtesy, do not do seat guarantee until you decide that you are flying that flight. I always book a few days before departure, and for all other segments, I pick ones that do not require seat guarantees. I will change flight and do seat guarantee when I am sure.