I've made a lot of progress, but the last week and a half has been eaten up struggling with a really aggravating bug:
Mission paths from the text field work 100% of the time, first time, every time.
Mission paths from the file chooser fail the first time, every time, and work after that. I have actually gone as far as booting up Wireshark to take a look at what is actually being sent via TCP, and the data sent in both cases is identical, right down to the line ending characters.
Completely baffled!