Any way to test the second mission of an arc?
Just rearrange them. You can drag the book symbols at the top of the screen to rearrange them. Then test your arc and your second mission will actually be your first.
Just be careful with this; unless they fixed it, I've had things in my mission change when I dragged it to another slot. For example, mission 2 has x mobs in a cave, and mission 3 has y mobs in a boat. I drag 3 to the 2 slot, and now I have y mobs in a cave, instead of x. Or I have x mobs in a boat. I stopped dragging and switching slots for this reason; it could be made a LOT easier to do. I think you should be able to select a mission to test play, instead of having to go thru the whole thing or having to drag and drop. Hopefully, they will improve this feature.
I've never had any problems rearranging missions. That's really odd. The next time this happens can you try to gather specifics?
What I have had happen - and this has been driving me bonkers lately - is that mouse clicks can activate pop-ups or button clicks in hidden (collapsed) details. So for example clicking on the "Front/Middle/Back" pop-up on detail B causes detail A's group to be set to "standard" instead of "custom". And sometimes these changes don't generate errors so you don't know anything changed until you go to test the arc, or examine the storyarc file manually.
And for a while things were cold,
They were scared down in their holes
The forest that once was green
Was colored black by those killing machines
Yes! I have had that same bug! Annoying, isn't it?
...without having to play through the first mission?