As a player, I would be very annoyed to learn that I couldn't learn Explosive Fireball directly and had to spend enough skill points to get Fireball up to skill-20 to learn it.
Neither of them are very good spells to start (see
http://noschoolgrognard.blogspot.com...-in-gurps.html for my problems with the current design of missile spells in GURPS) but Explosive Fireball is situationally useful for dealing with diffuse foes, and there isn't a good substitute for when you need to deal area effect damage at a distance.
I'm interested in the general idea of consolidating spells and making each spell more useful, but I really don't think that GURPS Magic needs more fetishization of skill breakpoints and it certainly doesn't need more incentives to create high IQ, high Magery wizards with a point in every spell. I'd suggest that advanced uses become available when you spend more than the minimum number of skill points in a spell, so they feel like something you had to spend time learning.
Finally, I don't think consolidating the Summon X Elemental and Control X Elemental spells into cross elemental categories is a good idea, nor is moving them into different colleges. Doing so eliminates the specialist fire wizard concept, which is rather iconic. It seems simpler to merge Summon, Control, and Create into a single spell, with Summon and Control as the baseline abilities and Create as the advanced ability.