Well Grover,
I suppose having had a look at your harness it doesn't seem to me it will give you a lot of support but it will give you the freedom to move. These harnesses aren't mad eto support you but made to prevent you from falling out of a tree! The talk the chap who is selling the stuff isn't to relyable (it is his product so always better then the rest) and him telling you, if I get this right, that one need support on level L3 seems strange to me as if the rest of the lower back doesn't need support. I do understand that if you are up in a tree you want to be as comfortable as you can to rest the parts of the body you do not use, e.g. legs. So in this the harness which supports you most is the best as simple as that. But realise a climbing harness is not made to support but a safety item and should be regarded as so. for back support; good condition of muscles (which should work as a corset to protect the back) and an elastic corset (only use when at work! in case you would need it) should do.
I hope you like this answer