Answering this question:
Airmanship aside, do I have to cross the departure end of a runway at 50ft?
as asked:
If it mean that you're asking if you should climb to 50 feet when there is no obstacle to clear, I would reply certainly not, if doing so means an initial climb at a speed slower than Vy. A Vx climb is a safety compromise in EFATO to get you over an obstacle. Do not climb at Vx if you do not need to do so for actual obstacle clearance reasons. Of course good airmanship, and neighbourly piloting is nice, but not at the expense of safety.
A single engined GA plane probably cannot glide back to a safe landing after an engine failure at 50 feet and VX, so why be there needlessly?