Is it true that most lattice degenerations (even in high myopes) of the retina remain stable throughout the lifetime and never lead to a rd?

Yes. Most people with lattice degeneration never have a retinal detachment. However in people who have had a retinal detachment, lattice degeneration is present in most.
Yes. Lattice is only detected on a careful examination of the peripheral retina by an ophthalmologist. It is not known how many have lattice because many (perhaps most) with no symptoms will not be examined for this condition. It is likely that most lattice is stable and causes no problems for those who have it.