So how do the new mandatory alliances play out when two allies declare war on each other?

[rageair]

In your case Scotland should join the defender.

 

[rageair]

Interesting.

I suppose it makes sense to base it off that, as there is always an aggressor and a defender.

But when peace is re-established will Scotland still have an alliance in place with the aggressor? Or will it have automatically dissolved the alliance?

The alliance is still in place.