Eliminating one option is not railroading; there could still be dozens of ways for the PCs to go.
The DM is not eliminating an option - he's eliminating a result. Obviously, and logically, if another ritual, other action, or even series of actions produces the same results in a way that was not conceived of by the DMs "plans", the DM is within his "rights" to nerf the idea. And to do so with some pretty heinous dishonesty.
The reason I find the dishonesty so unpalatable is that it violates one of the key jobs I see the DM as having - and that is he's got to have the trust of the players that when he makes a ruling in the game. When you tell the players what you're doing they should believe that you're telling them the truth. Because you should be.
The context of this IMO pretty much contradicts what you're saying in a key point- and that is that there aren't "dozens" of ways to go. There is one way - a way that keeps the "plans" of the DM intact. The train might jiggle back and forth on the railroad but it's going to stay on the tracks.