On the stats page, does the amount invested include the amount that the site is up? Or, should these two numbers be added to determine the house's total bankroll? Does the amount the site is up get debited when investors make a withdrawal or is it a running total that tracks the profits even after they've been withdrawn?
The invested amount is the same as the bankroll. If you are the sole investor, invest 100 BTC, then someone bets and loses 1 BTC, your investment is worth 101 BTC, and the site is up 1 BTC.
When you then withdraw your 101 BTC, the amount invested is 0, and the site is still up 1 BTC.
So "yes, no, and the latter".