OK that makes sense and is something like what I was guessing. So that's a lot of work just to allow a couple of trains a day in the summer from London
I'm trying to figure out how they could fit IET workings in, with most of the branch still being single track? Off the top of my head, the only ways I can imagine it being done are:
(a) The IET replaces a metro service in whatever hour it runs. But that seems unlikely because it doesn't fit with what you've said about the local service continuing to operate while the IET occupies the Newquay platform.
(b) The IET closely precedes or follows a metro service, so that both trains can go in the passing loop at the same time to make way for the service coming the other way. That would seem to require some extensive signalling work though to allow that.
Does anyone know what is actually being planned?