The final step is taking me much longer than the grid fill.
Some thoughts:
There can be no doubt about the axis of symmetry, particularly since you know the start and finish (or vice versa)
You also know 33 letters which must appear and 9 which are on a list of extra letters which are later going to be deleted..
There are some letters which only appear once or twice in the relevant area of the grid, so you know that these must be on the route, and so must their symmetrical pairs.
One assumes that the path is connected (in the mathematical sense and also common sense.
However, I am by no means there yet. This is becoming pretty tedious.