Does the cryo get pumped into the FCs on demand?
Yes.
That is, if more electrical systems are tied to it through a bus, does the flow of reactants increase?
Yes, there's a fuel cell flow indicator on panel 3.
If that's the case, would a lack of demand cool the FC to the point where you need to use the FC radiator bypass?
Yes.
Once I tried to blow up a hydrogen tank by leaving the heater on. The pressure pegged but at it didn't seem like I could get it to fail.
It won't blow, this isn't simulated (yet).
Is there no temperature indicator for the FCs, or is temperature just implied based on pressure?
There's no temperature indicator, just pressure and quantity.
As an aside, it would be immensely helpful for the checklist MFD to center the screen on the next switch.
I'm not sure what you mean, could you elaborate?
Cheers
Tschachim