There have been at least one study looking at this by Jacob Wilson, they had people work out with either a cyclic ketogenic diet, or on keto full-time. At the end they did a carb-up. There was a greater increase in lean mass on keto full time than with cyclic keto, they theorized that what you suggest here happens, there's greater wasting with the people being out of ketosis, Wilson said after a weekend carb-up they'd not be at a significant level of ketosis until thursday.

I'm suspicious about this, though. In earlier studies, high carb seemed to do better than low carb, but it turned out that they weren't making proper allowance for glycogen stores. If you fast somebody, or put them on a ketogenic diet, and then carb up, you get an overshoot of glycogen stores--so it's possible that the steady state keto dieters got more of this overshoot than the cyclic ketogenic dieters.

Do ketones spare amino acids? Well, yes. So does glucose. I don't know that this is an advantage of a ketogenic diet, so much as not a disadvantage.