I don't think there's ever a need to be so meticulous about freezing, you can easily claim the main character frag's froggy until you get the whole 5* and it won't make much difference.

About ninja exam, if you power up enough you can make it to the respective exams that give those scrolls, and once you do sweep only the necesary times to get all the new equips on your pos 1 (the rest is completely optional), of course you must make it to the higher possible ninja exam before trying to sweep. After all a single sweep up to exam 300 gives around 200k exp (maybe a bit less) x5 = 1m exp, not that bad considering that's what 5 of the last froggys give or what you get for 2 days of fully getting experience from any possible daily, which won't make you level up that easy.

Also i recommend you to lvl up to lvl 80 before starting to freeze, since you have unlocked already all treasures, functions and main character skills, the reason to want to stay lvl 78 when you can't take advatage of lvl 70 sage is out of my understanding.

Note that i actually dislike level freezing, and i don't promote it, but it kind of pains to see people who doesn't even know what they are doing. There's no much gain from it in the current game, and there's too much people doing it now for it to be useful.