I think you're off your rocker because I can't replicate that problem with my three Hi-Capas and I couldn't replicate it when they were bone stock either.

... or on a real steel 1911.

Oops, I meant #4 silly me. No rocker for me.
Half-cocked hammer can only be fully un-cocked by cocking the hammer fully then pulling the trigger; a feature found on real-steel 1911's. Try saying that three times fast :)

afaik, that isnt a feature on real steel 1911's as well
you should be able to manually decock a 1911 styled pistol completely and not have it stuck in the half-cock position
I believe WA is the only company that lets you do that with their airsoft guns
KSC tends to poke the valve a bit and vent some gas if you lower the hammer like that

anyway, it sounds a lot like fitment issues
when fitting the blowback chamber, rack the gun back and forth while screwing the allen key in the back in, there is a "sweet spot" that the screw has to be in; most of the time, it will not be screwed in super tight, but tight enough
leave it in the sweet spot where the gun still has smooth slide movement while at the same time the chamber being securely fastened

if this doesnt work, remove everything from the slide, make like a cabinet, and start filing