I am not sure I am understanding that last exchange, but a Scenery Item cannot be chosen as the target of an attack that would also affect troopers, be they enemy or allied. However, there does not seem to be anything saying you cannot target an enemy trooper with a blast that would also affect a Scenery Item. The be engaged with an enemy part of Martial Arts should be thought to also include Scenery Items, otherwise it would be impossible to destroy the AC2 in Looting and Sabotage. (I could see some arguing that Looting and Sabotage is the only mission CC can be used against a Scenery Item, but that would be a bit silly.)
I was surprised to discover that Martial Arts does not itself say you need to be engaged, base-to-base is the only requirement. It appears the only mention of Engaged is for CC Special Skills, but that has been clarified. Works out the same in the end though I suppose.
