You're probably better off maintaining the second license if you go ahead with your strategy of having the secondary work for multiple primaries.  It might be in use frequently, albeit for short periods each time.

But... if you have multiple primaries for the secondary, I don't think your switch of the secondary's IP address and alias is going to work as well.  What if two primaries fail at once?  It's unlikely, I'm sure, and this article is a good treatment of a reasonable, low-cost and responsible method.  Thanks!