Yep — probably just a coincidence, but still another piece of the puzzle worth taking into consideration. For me, it's not only about the festival itself, but rather the fact that it serves as a reminder that kites seemed to have been a fairly popular hobby at the time. There were large festivals, local clubs, and even stores dedicated entirely to kites. And when reading The Kiteflier, it's obvious that kite enthusiasts were often also interested in photography.I'd guess the Calvine photo is a deliberate, planned hoax.
It might have been a spur of the moment thing, using something passably "UFO-shaped" that happened to be there after noticing a military aircraft (or aircraft plural), but I think this is less likely: It would take at least a little time to hang the "UFO", and combat jets don't often hang around for long (except during airshows, photo opportunities etc.)
It could be a kite, but a smaller object/ model might be more manageable (and less likely to be blown about, so needing less tethers to keep stable for the photo).
The Glasgow kite thing is probably a coincidence; it's summer (and during the school holidays), it's when these sort of things most often take place. Lots of other people would have flown kites in all sorts of locations across Scotland that summer.
If the object is a kite, I don't think colour is an issue, it could be painted.
But again, this doesn't say anything definitive about the Calvine object — it simply opens up the possibility that we might be looking at a kite, and I think that such a possibility should be added to the list of candidates. Personally, I tend to lean toward it being some piece of junk that simply happened to look a bit like an angular "UFO," perhaps inspired by the triangles from the "Belgian UFO wave."