Can button be set to cause interrupt, and then it's functions changed?

[Art]

Yep Most of it will work the way you want it to on it’s own.
Especially since portb.o interrupt should cause wake up from sleep AND jump to the ISR where you can disable it.

[CuriousOne]

Can built-in timer used to count the time while circuit is on?

[Art]

That’s what DT’s Elapsed Timer does, yes, but if you want to use the portb.0 interrupt for a button as well,
there is a separate DT package. I forget the name of that, but someone will chime in. It allows multiple interrupt sources.