well after long term TRT or HRT usage... yes.. most likely you will completely stop making any natural test.
HCG isnt mainly to keep you producing test while on HRT but to keep stimulating LH and HPTA. which will in turn help your testes keep producing some and also be primed better for other factors.
i dont know all the scientific hoo ha behind it. but im sure someone on here does
as far as dosing... i know a few guys on HRT who run different doses as directed by their doctors... just like everyone runs a different dose of Test during HRT also. and like you run deca and most ppl do not
the dosings ive seen are usually 500ius shot 2 times a week
or 800 ius shot once a week
or 100ius shot once a week
and so no
now this is just the doses from ppl i actually know and have been on HRT/HCG for sometime