I have not mentioned Montero at all since every time you go to Verona, you can go to Montero as well. But you are right about it being almost impossible. In my current game I followed the start of the route: Debir-Scarlet-Rusty-Verona. But even with trolls at night I had to give up some battles in Rusty (but that does not matter since I have to return to Rusty anyway). Verona was even worse. So I got to Verona to get the Lady, but ended up doing only a couple of battles. Then I went for Bolo and completing that island was easy. After the return to Debir I headed back to Verona to do Verona-Montero-Verona. Going to Verona and then Dersu, instead of directly to Dersu (from Debir) cost me 4 hours, so the travel time will be 105 hours. (I try to follow the the route shown below - the currently optimal route) This time it was doable and even easy. A couple of levels and some critical spells made a huge difference. So I guess an optimal travel route is just that, optimal. It does not need to be possible
And by the way it is not even optimal. I found another even shorter route:
Debir
Scarlet Wind
Rusty Anchor
Verona-Montero-Verona
Bolo
Debir
Dersu
Uzala
Elon
Nameless
Reha
Umkas
Montero
Tekron
Rusty Anchor
Scarlet Wind
Debir
Sheterra
Dersu
Elon
Nameless
Uzala
Verona
This travel route is 101 hours.
Again, the first visit to Verona can be more or less impossible since you need to get the flying horse...