One reason I was told for why the attraction was moved to summer is because WDI believed that pushing to get this open quicker would force corporate to make decisions on other blue sky projects that had yet to receive a greenlit. Like the Beyond Big Thunder stuff and even Dinoland.
From what it appears, they did get permission to expedite Tiana's opening at least. Though it doesn't sound like it had much of an effect on the other projects they were hoping to get approved. A summer opening always made the most sense strategically though, so that's probably why corporate allowed the date shift.
I also don't really think Tiana has anything to do with a response to Epic either, at least not directly (the other projects on the other hand). This ride is not really being treated as competition to what Uni is doing over there.
Part of me wondered if the railing was temporary. Like maybe they were testing out lighting or some other new hardware or something. With the railing serving as both a temporary rig to support that hardware along with a safety feature for the crew working in that area so they don't fall of the side into the chute chasm. But the fact that they painted it green makes me think they may be planning on using it in a more permanent manner. Albeit I would expect that they intend to cover it with greenery if so. We'll see. You're right though, I definitely don't recall that railing being there.