So... Don't use it and instead use your class abilities and other mythic path features?
This brings us back to the initial point, if you don't spellbook merge, your mythic spell list is useless for any ability that has to pass a check. I don't want to use spellbook merging, but I also don't get to not spellbook merge and use spells from the spell list that has to make a DC against enemies you are facing at your level. There is only one possible choice, you either spellbook merge and use spells from your mythic spell list that has to pass DC or you don't and you can never use them. The only option the game is giving you is to merge spellbooks, which trivalizes the game.
Surely you get the issue here? Shouldn't be too hard to have something between "you are going to faceroll entire encounters, and can even one-round an enemy that you are not even supposed to defeat" vs. "you are never going to use this spell against any enemy you are facing". It would be an easy solution to just base your mythic spells based on your highest class level, but no, it is mythic level. Enjoy casting your spells from mythic list at caster level 3 or 4 for nearly entirety of act 3 and 4.