For one thing, the Apollo shield started in the very thin upper atmosphere, and they came in at an angle that meant they bled off as much speed/energy as possible in that thin upper atmosphere before going into the thicker atmosphere.
I don’t know that that makes a huge difference to the physics involved, though it certainly may have.
Of course it will make a difference. The whole challenge is about managing the heat build-up, which is the energy per second (i.e. power). If you hit the thin upper atmosphere you’re encountering less material, so less friction / pressure, so less heating. It means you can keep the heat on the heat shield in a manageable range, rather than putting it at a temperature where it would melt or explode.
the air cushion begins heating itself up instead of the object, reducing the amount of heat the object receives.
No, both heat up. The air cushion transfers its heat to the object next to it. At the kinds of pressures we’re talking about, you might even be getting nitrogen plasma rather than just nitrogen gas.
But it would also tail off as the bore cap heated, reducing stresses on it as it went higher.
If it went high enough for that to matter. If it disintegrated in the lower atmosphere it wouldn’t matter that the air got thinner in the upper atmosphere.
chunks of meteorites bigger than a meter have made it through the atmosphere, for instance
Is a metre the original size, or the final size? Also, reverse meteors (something starting with its maximum speed in the lower atmosphere) are doing things the hard way. Rather than getting slowed down initially by the thin upper atmosphere and then only hitting the thick atmosphere once they’re slower, they start out in the thickest atmosphere. OTOH, a meteor is a random collection of rock and metal formed by gravity in space. A pure metal plug cast on Earth is probably going to be a lot less prone to breaking apart.
the bore cap starting at the bottom of the atmosphere means that it’s likely it experienced less fracture stress, since the air would’ve accelerated with it rather than being static.
That doesn’t make sense to me. Something in a thicker medium is going to experience more stress. Try pushing a cracker through the air vs. through water vs. through gelatin. Which medium will cause the cracker to crack first? Obviously it’s the thicker medium.
Hit the gym, delete the lawyer, face the book.