I've done some research into Corridor and its nature for my own TNE spin-off game. As a caveat, I haven't done as much as many posters on this thread.
A lot of things don't add up about Corridor, but my impression of Corridor was when I looked at the economics of the area, it's poor - they don't have the kind of economic power like other areas of Imperium - the Spinward Marches, usually seen as the last true "frontier" is economically much more robust than Corridor. Types of economy that don't really show up in typical 3I economic analyses - interstellar trade - make Corridor even more poor - Corridor is pretty awful territory for the "small ship" J-1 merchants. In fact, even accepting the reality of the huge "container ship" traders that would logically exist in a "big ship" universe ... there's not much reason to send such superhaulers through Corridor on their way to the Marches (with stops along the way); the Marches are pretty economically independent and the transit fees of shipping stuff from the rest of the Imperium to the Marches can't be economical.
So haulers would target Corridor worlds themselves, but again, there isn't as much peripheral trade (from the hub worlds out to the worlds that'd be served by feeder lines) because Corridor is sort of sparse. In other words, there's trade money coming in but it's not like the trade money elsewhere in the Imperium. It's noted in some of the MT material that the Imperium deliberately discourages self-sufficiency on the part of member worlds; it wants worlds dependent on other worlds as a bond to hold the Imperium together (iirc that's from the Hard Times sourcebook). So there's going to be some trade, but not as much. This interdependence, while great in times of peace to hold the Imperium together in a sense of community, it puts a dent in the ability of any given world to actually build their own weapons and fleets if trade breaks down - a world has lots of resources, enough to become self-sufficient given time. But time isn't something on the side of worlds in the Imperium the Rebellion era and it takes time to develop them when you've been totally dependent on off-world sources for as long as anyone can remember.
The Corridor Fleet itself doesn't make much sense - do you really need such a powerful fleet to deal with admittedly numerous but lower-tech bandits? Not really - more cheaper and smaller ships is the way to go with dealing with handfuls of Vargr raiders with no central coordination.
My conclusion is that Corridor is the Hadrian's Wall of the 3I (or "was" for my purposes). It didn't need to be done that way. The Imperium could have done things different and likely more cheaply and it'd have been as effective (or moreso). However, the Emperor likely wanted to show the might and resolve of the Imperium, not the Vargr (though I'm sure it probably discouraged many Vargr who might have flirted with the idea of raiding), but mostly to the Spinward Marches. It wasn't that the Marches were subversive nor was the Corridor Fleet a handy way to stash reserves against the Zhodani. It was to show the people in the Marches that "Yes, we're with you. We'll back you up. We're all in, today, tomorrow, and forever."
Given how poor the Corridor area is, their local fleets were probably understrength at best; many places may not have even had them (it's my opinion that while Imperial rules may have required a given system to have a local navy of X size for local defense and so on, actual compliance probably varies/varied pretty widely). That was fine while the powerful Corridor Fleet was around.
The Rebellion rolls around, and Lucan pulls the fleets out of Corridor; given the dire need for ships, the relative lack of value in Corridor, the self-sufficiency of the Marches, and Lucan's results oriented personality, it's likely that when Lucan pulls out the Corridor Fleet, the main fleet, the reserve fleet, in fact any fleet where the Imperium footed the bill - which is likely a lot of ships. Yes, it'd be very unpopular with the people in Corridor, but Lucan seems to be more the type of, "I'll take these ships, knock Dulinor out of the war, then we'll come back and mop up the Vargr."
As for the Depot, to me, without the fleet to service, it's likely anything that isn't bolted down in the Corridor depot is taken. In fact, by the time the Corridor fleet is taken, things have gotten pretty dire - they probably took a lot of stuff that was bolted down as well (with all that cheap fusion power and anti-gravity, the definition of "bolted down" doesn't encompass as much). A lot of those even "fixed" facilities would be pretty useful in areas where the fighting had destroyed it all. The Vargr poking around don't find a Depot with a massive contingent of mothballed ships. They find an empty, abandoned depot with lots of mostly scrapped husks; everything else including empty hulls that might be useful somehow were probably towed off. The idealistic answer would be remaining ships would be served by local resources, not by the Depot, the reality is that those systems would just have to "make do."
they would be called up and on active duty status. Crews could rotate, but, those ships are going to be available 24/7/365.25 ship shape and Bristol fashion!
o: Their admiralty needs some serious work.
