pointless busywork designed to justify a flow of money
Ding ding ding! There is no reason to expect the organizational structure and goals of the NSA to be any different from middle management at Microsoft, except unlike Microsoft, the NSA doesn't have a finite money supply dictated by the market and a CEO that would be willing to draw the line somewhere if enough money was squandered. I don't know what performance metrics they use at the NSA (doubt it's stack ranking), but at the end of the day it's going to be about helping your boss convince his boss that you should be awarded a bigger budget because what you're working on is really cool and will impress the guys upstairs.
My guess would be that in any of these instances, no judge would rule that you must keep updating the canary. However, I'd imagine that they might rule that you broke the law by setting up the canary in the first place. Of course, there's an obvious problem with that -- as long as you never get a secret warrant, you clearly couldn't be prosecuted for violating one. So it's a weird situation where an action that is otherwise legal, becomes retroactively illegal upon receiving a secret warrant. It's a bit of a mindfuck.