A1 dives to the floor after a loose ball. He retrieves it and is lying face down on the floor. With both hands on the ball, he rises up enough to pass the ball to his teamate. He nearly approaches the kneeling position before the ball is released. However, his knees never left the floor nor did they ever slide. Is this different than the opposite situation where the player gets the ball lying on his back and he rises to a sitting position? The latter situation is of course legal but how about the former situation? I had a no call, caught hell for it, but still tend to believe I may have gotten it right. Yes or no?

I had the same situation last week, and no-called it. I'd be interested in a definitive answer from one of the experts.

Sounds to me like an attempt to get up, even though he stops and passes the ball.

If he's on his back, he can sit up, correct? What would be different about getting to his knees if he's prone?

He can sit up. But, it seems, that is the case only if you are on your back.
Case Book 4.43.5 B pretty much spells it out.
Original post states "nearly approaches the kneeling position". Probably a HTBT to see if in your judgement was an attempt to get to his feet.