Its a longer thread than it should be OP, grant you that. Reading your first post, that is a typical way of supplying a garage, albeit with a couple of slack things that could easily be rectified. So I think you'll be a hiding to nothing to suggest the original installer has done things incorrectly and should install new. I get the feeling, you had hoped we'd all agree that the builders of your property had installed the supply incorrectly, and you could gain some evidence for your cause to have an upgrade FOC?
As regards, your new additions, you can add as many sockets and lights as you want, the supply is limited by the 13amp fuse in your FCU in house, as been said countless times.
Unless you intend to run high load equipment in your garage, jig welders, hot tubs etc, I can't see it being a problem.
The 1.5mm swa cable has a ccc of 22A (Ref method D buried in ground/ducting), you could re supply that from the inside of your house, if limitation of 13A, is an issue for you? It seems you have been given various pieces of good advice, but not what you want to hear.
Why not get some quotations, and discuss with a competent electrician.