As you have found FrSky do sell FASST compatible Rx to work with your Futaba and I have used exactly the same set up with some of my boats.
If I understand correctly you are using five devices to control your model
a) rudder servo
b)Main Sail drum
c)Esc for motor
d)Jib servo arm
e)Fore Sail drum.
To understand further we need to know which Rx channels are attached to which device ie a) to ch1 etc. If you are using Y leads please also indicate the connections.
Secondly what exactly are your drum servo types?
I have a Cariad and use a Hitek large main arm servo to control the mainsail and jib. A rudder servo and an Esc for the motor. I did fit a separate servo to control the jib and fore sail and whilst it worked it was very difficult to use in practice at any distance from the shore so I removed it and now just allow the jib and foresail to set at their own pace. I do have a shroud to adjust the set.
If you are using the Mtronic ESC to power all those servos I suspect you could be drawing more current than its design and might need to use a separate battery or UBec with higher capacity. This might be the cause of your lack of motor control if the ESC is shutting down under load.
Sorry for the long post but hope we can help if you provide the info