The sequences of the thermally induced martensitic and intermartensitic $L2_{1}\to 10M\to 14M\to L1_{0}$ and $L1_{0}\to 14M\to 10M\to L2_{1}$ transformations are observed in Ni$_{50.6}$Mn$_{28.5}$Ga$_{20.9}$ single crystal on the cooling and heating branches of a thermal cycle. The modes of stress-induced intervariant transitions in the 10$M$, 14$M$, and $L1_{0}$ martensites are investigated on the basis of matrix approach, and the stresses of orientation-variants’ rearrangements are determined as a function of martensite structure, transition mode, and temperature. The twinning stress decreases along the $L1_{0}$, 14$M$, and 10$M$ martensites with a decrease of period of a nanotwinned structure.