The $450k ‘down payment’ could have been the equity from the sale of another home, basically a roll over. I think it is pretty common to use the proceeds of one home sale for the purchase of a new home.
If the OP put down the value of her home as $750, I don’t think the schools are going to go on Zillow and put down a different value. Some schools get 30k applications and they just don’t have the time to google every entry on the CSS.
It doesn’t really matter WHAT the school considers as they have the right to consider just about anything. Once the OP determines that the school has used the correct numbers and has awarded the correct amount of aid, and the OP feels it is not going to work, it is time to move on. This school is too expensive.