"1) If I were to walk up to an empty BJ table with the rack covered and locked, would I normally expect that rack to have been filled to approximately what the casino considers a full rack? Or, is it common for a casino to close a table and just leave the rack as it was when they decided to close the table?"

No, floormen look at a rack and deside if it is OK or it needs a fill. But they are probably close to full.

"2) Assuming that your answer to the above question is that the rack would be full, could I then assume that whatever the distribution of different chips were in the rack would be the approximate percentages I could use?"

It would be simple to look at a rack and count what was in it. If you looked at a couple closed tables you would have a pretty good idea what they were using as a full rack.

3) What I am trying to do is confirm my belief that on average, I could expect there to be less $2.50 chips in the casino than there are of any of the other denominations $25 and down, and perhaps even $100 and down.

I would agree with that, as there is no need for $2.50 chips on the specialty games, craps, roulettes, etc.
I've never been involved ordering chips at a casino that used $2.50 chips in the rack. I did deal BJ at the Nevada Hotel for 3 months in 1980 for about 3 months. We might have used them but that was way toooooo long ago to remember.