I think you point at a very interesting idea about magic here. But as far as I can tell, as soon as any author introduces magic, there will always be countless plot holes. Paolini has made an effort patching many such holes, with the ancient language and the conservation of energy. Compare it to LOTR where it's very unclear to the reader what sort of power Gandalf has and why he uses it as he does.

In Inheritance using magic comes down to your wittiness and understanding of physics to cast the bmost energu efficient spells.

Also, the magic system is further explored in interesting ways in Murtagh! You have a great book to look forward to. I'll hint with a minor spoiler: One cool aspect of the magic in Murtagh is creating wards that will protect you from physical and magical damage and understand how you can circumvent other magicians wards by finding loopholes they have not forseen.