I'm hoping this hasn't been previously covered, or that I'm just not using it correctly to start with, but once I have a saved route, while pinching, squeezing and swiping my screen to move around the chart, I find it desperately frustrating that I keep accidentally touching the route line and dragging a great big diversion into it, necessitating an 'undo' to put it back where t was.

Not a very popular idea then I guess lol.

While you're planning, like you say, it's easy to undo if you accidentally change your route. While you're flying, the route is indeed locked as you suggest to prevent accidental modification (because there's no undo button).

It's NOT locked in Nav mode (ie after pressing "Go Flying") on my iPad and I have encountered same mild frustration at having to select the unwanted waypoint and delete. When does it lock - once moving? Certainly not locked when stationary?

I support the idea of a locked mode.

Yes, once you start flying the route is locked.