Clearly this is not a mistlands issue. If mistlands was completely different, it would still block you in the same way. Why aren't you making the same claim about plains? As you mention it's blocking you as well.

You got bad RNG on this one. That happens, but it sure looks like you used the map viewer to make your picture, so you must have seen there are other options available.

Also, if you're ready to fight Moder, you should be ready to enter the plains. Obviously it would be best to not have to land your ship in plains, but you should be able to cross plains.

Unlike swamp, plains is actually generally not that hard to land in either. You can clearly see your enemy from the ship, so you can find a place to land that should give you plenty of time for confrontation if necessary. Just be careful leaving your ship there, of course.