Why did I, again pure Zetetic observation, feel the acceleration?
I've said this before, but I feel you misunderstood. The term "feeling acceleration" is extremely vague - it's a term you've created to make a sensation seem intuitive. What you actually perceive is your acceleration relative to the air around you. Because motion is always relative, this is exactly the same as the air accelerating relative to you - there is no universal frame of reference from which you could distinguish the two. That is the sticking point - you feel "the acceleration", but there is no objective answer as to what's accelerating relative to what.
Because motion is relative, you could also completely rephrase UA to mean something like "anything other than sufficiently massive celestial bodies is accelerating downwards at 9.81m/s
2 relative to the otherwise stationary bodies". Physically, the two are one and the same. All that changes is the frame of reference you chose.
Perhaps a different thought experiment will help here. Try to imagine the sensation of your body being held down in a river, well under the surface. Imagine how the water would feel against your body. Now, imagine a separate situation. You're submerged in water which is not flowing, like a lake, and you're being pulled through the water.
Without external information, you would not be able to tell the difference by the sensation alone. This is exactly the same here.
And yes, your comment about Einstein is correct. According to the Equivalence Principle, it MUST be the case that you won't be able to tell the two apart. Basic physics would break if this wasn't the case.