If the shader replacement is causing issues, give the knockout page's slammin' hammer a go.
Slartibartys build?
With garrys mod specifically there is some issues with that. As it only works with SDK 2013 and therefore uses SDK 2013 dlls, it cannot read Portal 2 and newer .vtf files (v7.4 vs v7.5), unlike garrys mod which has the vtf.dll updated.
I though about running the patched hammer from GarrysMod/bin, but of course it does not even start.
Your build did start when ran from GarrysMod/bin, but it did not find the editor materials (I got that 4 axis grid thing in each viewport, usually solved by having hl2 mats etc in gameinfo.txt), but when ran from SDK 2013 it worked just like slartibartys build. But then again there are the above limitations.
(And both slammin hammers crash now when opening model browser ¯\_(ツ)_/¯)
Anyways, it was worth a try, but can't have everything I suppose.