If you ask a Wave sailor if the reaches are boring, you will not get that answer. That is the reason the designated class course calls for two upwind legs, two reaches and one downwind. They hate downwind.

Two finish lines is plausible but I think not necessary. For example if the H16s are tacking downwind they might cross the line on a broad reach on starboard, having rights over the spin boats on port. However, both would be able to see each other and do something about it.
If the H16 was on port on a broad reach they would both be going the same direction (keeping in mind the spin boats would also be on a broad reach, not a screaming reach (diagram is not perfect).
The Waves will probably not even be in the area (either already finished or farther back. But even so, they would be visible to all downwind sailors on broad reaches, spin boats or not.

By having one finish line, we would have the start line ready to go and could get races off faster. Otherwise we would all be waiting for some very slow boat way back having to finish through the orange finish, and then reset the start mark after he finishes. More time consuming.
From my own experiences the worst RCs are the ones that fiddle around forever between races while I am bobbing around. That is why I always try to be a stickler about being prompt to start races and have as little dead time between races.