There is no rule of thumb about how high up above the ground the bottom branches should be. Basically what ever appeals to your asthetics and what you feel is good for the tree.

Keep in mind though if you are building a parapet, that you will have to taper it towards the top to allow light to reach the bottom of the plant(s). Or else your trees will lose the lower branches naturally due to low light.

Walking along the other side of the hedge over the weekend I noticed how many of them had a 2nd and ocassionally 3rd or 4th trunk clustering, making for a dense hedge. The browning observed may be exasperated by such crowding.
So would removing all trunks but one, encourage a denser, healtier tree?

Are there any opinions if these plants are both trees and shrubs?

If there are many and especially if some of the 'extra' trunks are just suckers removing some would help the health of plant. More air and light circulating would get to the trees.