Can someone explain why the "odds against" have a "+" figure but then when you click it, it switches to "-" ?

Also, is the number directly underneath the line the amount of mbtc available for betting?
Hi. That is because you bet "against".
I am not used to american odds, but basically, if you bet against an event, it means you are the "bookmaker" (the house), so this is the opposite of a normal bet you place at bet365 or 5dimes for instance.
As explained earlier:
Back (for) at odds of +200 is the equivalent of Lay (against) at -200.
When you place a bet "against", Fairlay convert the odds to the equivalent of a back bet.