I do not care for the evade option, mostly because it is chancy. Here is my two cents on what I feel is a better option.
Leave the idea of the channeler being in hiding in a city as it was - and if the city is destroyed he automatically escapes to the nearest city. But if he is in hiding he obviously cannot help defend the city either.
For those who want to have a battle mage but still have a safe option.
For heroes other than the channeler, I like the evade idea just fine.
For the channeler, there is a unique spell only learnable by sovereigns called Get My Ass Outta Here. It is only castable in combat and only castable during the first round of combat. 100% success rate, cannot be interupted etc etc. The Channeler is teleported to his capitol city.
This I think is a great balance. If you are in a battle you are able to assess the odds. The presence of the channeler alone should make otherwise even fights a pretty easy win. If the odds are bad even with the channeler there, then obviously the army is going to get creamed if he bails.
I think there are some good payoffs and penalties for this option. If a channeler ports to his capitol at the drop of a hat, then the player is going to have togo thru the hassle of getting his channeler back to the army if they survive (or building a new army if they don't. It's an escape with a pretty hefty cost.
What I really like is what happens if two armies fight and they each have their sovereign with them? Say it is fairly evenly matched...there is going to be a very strong temptation to 'play chicken' and see who blinks. Yews you are putting the entire game on the line, but if you stay and the other guy runs, then what was an even battle now becomes a slaughter. And if both channeler's take a chance - it is an Epic Battle To Stay in The Game!
It may seem similar to the evade skill, I like it better as an unfailable spell because there is no RNG, and even in a 1-on-1 (which with evade you say he probably won't get away) with the spell he could but again you have the hassle of losing position, you could even tweak it to either cost a lot of mana or possibly a dash of essence. You could also have it affect his fame, i.e. if you ran too often you would get a reputation as a coward that might affect diplomacy. (In other words you still want to give them a reason to just hide rather than make a half hearted attempt at a battle mage). Give people a sure way out but there has to be associated costs, so that if you really don;t have the nerve to put your channeler on the line, you are better of leaving him at home.
Just so people don;t complain about "forgetting" to warp their channeler home, there should be an option setting so that every battle that has your channeler involved prompts you with the "Do you wish to cast Get My Ass Outta Here, m'lord? Y/N". Also I think hiding in a city if the city falls it should be a guarantee that the channeler flees to the nearest city.
Obviously if you are down to your last city, you better have your sovereign out fighting.
I think too many people look at it from "my channeler is going to die" and not from a "Yay! I defeated the enemies sovereign! What?! He teleported out at the last second just as I defeated his massive army? WTF??" Continuously having to track down and fight some guy who has sunk max skill into evade ability is going to be nothing but frustrating. (and honestly from the evaders perspective how much fun is it going to be always on the run just because oh noes my channeler might die!")