If you read what I wrote, I specifically state that it is regardless of those other restrictions, that having more 'mega-nukes' still increases the frequency at which you can deploy them.
I read it but it's wrong. Having more high-level spells means that you can actually use them. In the current implementation you can't use many high-level spells at all. So you don't increase the frequency, you increase the arsenal, the number of spells you can use in combat. And yes, I think that this makes combat more fun, not less.
(To decrease the frequency of spells you can use you had to decrease cooldown times.)
If increasing that number comes at the expense of multiple smaller ones, it is a choice to make.
Yes, but a bad choice. I really don't think that the "choice" which spells to take into battle at the cost of others is a good choice. It's a choice that makes combat less fun and flexible and a choice that only leads to more save and reload.
If both cost the same, you take the bigger ones, because it'll diminish your flexibility to a much smaller degree.
And that's where you wrong. Smaller ones aren't worse because they cost less AP and have way shorter cooldown times. The best arsenal has both high-level and low-level spells and skills because this strategy gives you maximum flexibility, especially for longer encounters.
But for a very high-class character with maximum points in a specific school it's imo ok and good that this character can have an arsenal of many different skills and spells.