Do you actually put these new tech-trees straight into a MOD folder (and, then activate that specific asset from there with all its new & necessary files) as it should?

There's your problem probably... defaults shouldn't be replaced or altered directly - it's best to leave those alone and provide whatever 'other' trees from a MOD folder only.

Necessary? If one race (be it custom or otherwise pulled from a Total_Conversion schema, etc) is meant to use a specific new tech-tree, it must be declared into a modded raceconfig.xml file also. Again, from the MOD folder.
Everything else could be a number of files which alter many assets such as graphics & pre-requisites found exclusively into the new tree; since, special trees still have to tie themselves with the main_core_general TA tree.

I'd highly recommend the tech-editor (which you seem to have done) to create it from either, scratch or by simply editing a few lines to 'test' first. Extra care with 'typos' or accurate values should be made, i might add. It only takes a single missing backslash anywhere to screw it real bad and prevent it from loading or be detected by the engine.

As a general rule of thumb, any modifications **MUST** be stand-alone, called up from a MOD folder and compiled (or edited) in such a way that every necessary elements are available from it.