OP: I know a lot of people think the idea you put forth is crazy, but in a way I think it's very plausible, primarily due to the very PREGNANT belly AND the fact that Kate did not walk or act like a woman who had given birth to her first child only 24 hours prior.

A woman will have a loose flabby belly but no way will they have a well-defined pregnancy belly. And almost every woman who has her first child via natural childbirth will be walking a little slowly/carefully at least for a couple or so days.

What I do not find plausible about your theory is how many people would have to be "in" on the hoax and could it be done. Also, why wouldn't she just have stayed in the hospital until the actual birth? But moreover, why wouldn't she have worn something to "hide" the evident pregnancy belly?