Since I resolved this, I may as well share what happened.
First, a few important callouts:
in my AppDelegate (which implements WCSessionDelegate too), at the top of didFinishLaunchingWithOptions, is where I have the boilerplate:
if ([WCSession isSupported]) {
WCSession *session = [WCSession defaultSession];
session.delegate = self;
[session activateSession];
}
Then, in my ExtensionDelegate (which implements WCSessionDelegate too) in applicationDidFinishLaunching, I have the boilerplate analog for the extension:
WCSession *session = [WCSession defaultSession];
session.delegate = self;
[session activateSession];
There's no if statement required here because the watch always supports WCSession.
So where was the problem?
I have an InterfaceController which has a label to indicate if the phone is reachable (while I'm testing).
In the willActivate, I was
a) pulling the delegate from [[WKExtension sharedExtension] delegate] and setting a view property for this controller (because if the ExtensionDelegate is also the WCSessionDelegate, it needs to have a reference to the screen that needs the data, or at least, that's how I'm thinking of it.
b) calling [[WCSession defaultSession] isReachable], but in this case, it appears that the willActivate of the main InterfaceController of a booting app will actually fire BEFORE applicationDidFinishLaunching (which, in retrospect, makes some sense).
So, long story short, if your app isn't running on the watch, and you're using WCSession, be sure you are not using it pre-emptively (I moved my snippet to awakeWithContext which works just fine, and is actually the proper place). Also, you may also wish to always add a check for isSupported to make sure you're not doing something when you shouldn't be (this appears to be the only thing that you can use before calling activateSession to check if things are good to go, and you can see I'm not using it above, for efficiency's sake.
Anyway, yay. Also, if anyone has a better way of doing any of the above, please share.
