Eomer could certainly be exclusive to the Exile camp. The reason we didn't do this is that he does feel like a core Rohan hero, so we weren't sure if you should need to have an outpost to build him. But if you'd prefer him only in the camp, we could do that.
The buff is longer than usual because we wanted it to be easily usable for fighting and aggressive maneuvers - it shouldn't already be half gone by the time you travel from the camp to the enemy (whereas statues are meant as a more defensive bonus and are also cheaper, hence their shorter duration). Other buffs like the warg howl or activated hero abilities usually last 30 seconds, but those can be used when the fight begins. As the exile camp also must account for travelling time, I think it's reasonable that it's longer than that. Though we could shorten it to 45 seconds perhaps?
The 30 second standard is actually something I've been meaning to get some opinions about. It's the length we currently use for most buffs like the Howl, the Iron Hills Guardian ability, and so on. Do you feel that is a good length, or do you think the buffs have too much uptime that way?
Banishment to Exile must carry a strong penalty, I think, and I really like its unique drawback, so that is something I think we'll keep.