Einstein's "Photon in a Box " is fatally flawed. There can be no recoil by the box when a photon is emitted. The emitting electron simply drops to a lower energy level to lose (-e = -h) where (h) is Planck's Constant. The photon has an energy (e = h) and so energy is conserved at the atomic level with none left over for box momemtum change.
Then when the photon collides with the box'es wall, only one of two things can happen:
First, it's reflected but there is no energy left for transfer to the box because this would require the reflected photon to have an energy less than (h), the indivisible minimum Planck value.
Second, if the photon is absorbed, the absorbing electron gains energy (h) and the photon is annihilated for an energy loss of (-h). Thus all energy is conserved with, again, none left for transfer to the box.
Therefore, the box could never move.
Not only the above, but the entire experiment is pure sophistry and quite unnecessary to its supposed purpose. From it, we wind up only with the posit of photon mass (m = e/c2 = h/c2) and (e = h) for photon energy.
Obviously, (e = h) is not a general expression of (e = mc2).