Regarding the conquest discussion:
An attacking unit being able to capture installations definitely makes sense, but I feel like there should also be a chance capture portions of the population, if one exists.
In regards to surrendering, I think it might make sense if there was some chance every set period of time that one side in a ground conflict surrenders, with the chance being based on factors like the current fighting strength of friendly units, on average, (compared to their fighting strength when undamaged, at peak performance, etc.) the current fighting strength of the least damaged friendly units, on average, how many friendly units are present that have little to no damage, etc. while comparing this to a similar analysis of the enemy's strength.