Is there an extension available for a 10 foot door?
No, this will not work on a 10-foot high garage door. The extension kit will only work on up to an 8-foot high door.
Will my existing Craftsman wireless keyless entry pad (PN 9-53876) work with this opener?
No, it will not.
I currently have the Geine ISL 950 but the motor stopped working today. Can I simply purchase the Mach Force Plu 2 HP Premium Screw Drive Unit and only install the Head Unit and use the current track etc?
No, the track for the ISL 950 will not work with the MachForce powerhead. The Machforce will come with the new track to use though.
Can the motor on a screw drive opener be slowed? The door that this is on is one piece and very light weight, mostly translucent plastic panels with a lightweight wood frame.
You will be able to adjust the force for the garage door opener. See page 17 of the owner's manual.