I always saw the TW conduits as permanent or at least semi permanent mainly b/c of the 5's map of the conduit system. Don't know how accurate it was, but it'd be hard to make a map of something if the objects in the map were always in a state of flux
You're assuming too much continuity. In TNG transwarp was just a "superior power source" that could go faster and longer than warp. The Enterprise was able to keep up with the Cube for awhile in Best of Both Worlds for example and stay within striking distance. In First Contact one assumes it was the same given the Federation fleet was able to still intercept the Cube rather far from Earth to engage it in a long running battle(unless you choose to believe the Enterprise E got from the Neutral Zone to Earth in a few hours).
By the time the Borg came to Voyager in Scorpion, they were still using the "superior power source" mode of travel. Otherwise the "time to travel through our space" argument is just two fold stupid, the Borg for proposing it and Voyager for believing it. Otherwise they should've just been able to drag Voyager to the edge of their space, heck drop them off at Earth for the cure.
It wasn't until Dark Frontier they started changing things to suit the various plots of the week. There was the transwarp coil that apparently anyone can just walk onto a Borg ship and grab. By all accounts it functioned similarly to the "quantum slipstream drive" that was used in Timeless which Seven supposedly adapted from Borg technology. It used the same tunnel graphic too. The Delta Flyer was able to use one of these to fly to Borg Space and back in the space of one episode.
In Unimatrix Zero by all accounts, they just seemed to have forgotten they had this technology as suits the plot as Voyager was able to intercept Borg vessels at will... except in the same episode they retcon this again since the "rebel Borg" ships joined them in a facepalming scene.
It wasn't until Endgame that transwarp hubs with exit apertures were brought up. Which given the way the Borg and Voyager had already used "transwarp' to fly from non-fixed destinations. Heck they even maneuver while at transwarp. The whole reason of being for the transwarp hub was a plot device designed to be "important" enough to give Admiral Janeway an excuse to martyr herself to destroy while simultaneously getting Voyager home with the Earth exit aperture which apparently the Borg just didn't feel like using in the two previous attacks.