“Besides . . . an interesting number of credible folks say such assumptions are just plain wrong.”
They are not assumptions. Mass increases as one approaches the speed of light thus requiring more energy which then increases the mass and on and on till mass becomes infinite. I don’t know what credible folks you are referring to, but the most credible physicists, including Einstein accept this fact. The ball bearing experiment you keep quoting is about inertia anyway, and highly questionable.
Considering one’s self smarter than Werner von Braun
has to constitute one impressively huge pile of stinking humility! Congrats.