The national security thing is a red herring as the underpass leads to "outside the wire" - ultimately to MOD housing; people can access the housing without using the underpass, and many do in order to cross the Almond by the old bridge.
There seems to be no concern over public access to similar estates at Dreghorn and other such MOD accommodation across Scotland.
I'm not a structural engineer but my decades in construction tell me that there can't be any safety risk to pedestrians or cyclists from the state of the bridge and if there was a risk to the bridge from foot traffic then the risk from vibration caused by traffic passing below would massively exceed it,surely.
Could a temporary path over the grass down to the underpass not be laid at minimal cost and accommodated within the security concern to allow use in an inward direction? I'm sure pedestrians will walk down there anyway rather than add a mile and a half.