It looks like we have a tie over where to send Myark. So let me go over why you should send Myark to the Desert.
1.) The Desert is the safest place for Moskurg's heir. It was the place where they lost the least last turn, so chances are they're going to try to send their heir there. It would be extremely dumb for them to send their heir to any place else, as the desert is where they're most likely to win.
2.) Moskurg will likely send Al-whatshisface to protect their heir in the desert.
3.) If Myark and Al-face meet, then Bjorn will be safe and sound in the Plains where we have the greatest pre-explosive ammunition advantage + Protector advantage + Expensive Restless advantage. Meanwhile, we will have new advantages in the desert (same as Plains - Expensive Restless, cheaper cannons, and the Protector which should excel in the Desert terrain) that should ensure that Myark will win over Al-face, and thus ensure our victory over the desert, denying Moskurg the revision credit.
4.) If Al-face is sent to the Plains, then Myark will almost certainly ensure victory over the desert while our advantages in the plains means that even with Al-face there, Bjorn will almost definitely survive if not win.
My point is, the Desert and Plains should be places where we will win fairly easily. Most of our designs this turn help in the Desert and Plains the most - the Protector will work the best in the flatter terrain. The Expensive Restless will be a lot more effective at transporting men and ammunition in the terrain. Our now-Cheap HA1s and now-cheap-every-other-cannon have always been best in the Plains and Desert.
With Myark facing Moskurg's heir, we'll definitely deny them the revision credit. With Myark facing Al-face and their heir in the desert, we'll still have a significant chance of denying them a revision credit while practically ensuring our victory of the challenge in the plains with unopposed Bjorn.
TL;DR: They will almost definitely have their heir in the Desert. But if they have their Hero and their Heir in the Desert, we will likely lose there. Sending Myark there will greatly increase our chances of hurting their Heir. We don't need Myark to protect Bjorn, as we will already have a great advantage in the Plains. If they do send their Hero to the plains, we will be able to easily take their heir (again, it'd be idiotic for them to send their heir anywhere else) and the desert while they have a much smaller chance of even wounding Bjorn.
Also remember that Myark has the wand of true light, which is a really powerful theatre-scale weapon.