I am hesitant to unhook lines at the abs module as I don't want to introduce air into it.
You already disconnected the lines at the master so air has already gotten in.
There would be no difference disconnecting at the other end of the same line.
You have stated that when last you bled the system, you got no fluid at the front calipers.
You also say that you don't want to be screwed and sold another master but you don't want to prove that the master is working.
Electrically unplugging the ABS/HCU module SHOULD have done nothing if the master is not working.
I do agree with you about getting screwed.
However if I was still doing this professionally and working on this car.
I would replace everything just because you worked on it first.
Mainly because I would not wish to be responsible for something that someone else may have screwed up.
One thing I never took lightly was repairing brake systems.
Dave