donut king,
Remember, the manufacturer has no powers to arrest, and the flight manual is a book that manufacturer writes (hopefully a non-fiction one!)
The law says you must obey the limitations, and you must perform the necessary performance calculations (see FAR 91 or its equivilent).
If you operate for hire, your company procedures manual is often required to be obeyed, so those might be mandatory, as well, and they often wrap up major parts of the flight manual into it. These procedures are sometimes mandatory. Thus, for hire there is some possibility that the recommendations are mandatory.
Regarding Vtoss, CDP, etc. these are needed to perform Cat A procedures, but I know of no operator who is required to fly them (help me here, ppruners!). Thus, you just takeoff, cruise and land as you should. Often, operators are required to load to Cat A enroute weights, so you are engine failure proof for 99.9% of the flight, and take a slight risk during takeoff and landing.