First I would like to thank you for this great project, it really made it possible for me to start playing with the GeoConvert tool, and understand the process.
Now, I can fly in the South of France, thanks to you!
I tried it last week-end and I got the parsing issue you just fixed on FSCloudPort aiport coordinates. I was about to send you the patch but you were quicker..
Anyway, having looked at the code, I found that the stitching process does not run (yet) in parallel when multiple stitched images are generated. I had a little free time, so I played with the idea of implementing it. I attach the patch and the resulting source files for easier review (TileStitcher.cs in particular). Feel free to incorporate this change into your code. On my not-so-fast-computer, on a level 9 area, it went from 14mn41 to 5mn36. So it looks like a worthy optimization.
I am really looking forward to the rest of your (ambitious) roadmap.