Nitrogensports max bet increases if the match is starting soon like less than a few minutes or so. You can see the second parlay that you linked the 1st and 2nd leg is 5 minutes away from starting. This only applies to single bets though. For parlays I think the limit is up to 50 BTC not including the risked amount.
I believe so, because when betting on parlay I don't see a limit and maybe I did not reach my limit since I don't bet over 1 BTC for a parlay.
The betslip that was shared was pretty normal in Nitrogensports, that how people trust them, they are comfortable betting that big amount even if they play anonymous.