hammer not re-engaging in the cocked position is most likely due to the sear not being aligned at all. take the assembly housing out again and re-fit it all. usually that does the trick.
reason:
when a whole assembly goes back into the frame (which is only held by that pin or 2, dependent on brand) it's likely to have slight mis-alignments when you fit it back into the exact sized slot since you have to wiggle it back in there.
as with other worn parts, you may also want to check the other sears. since there's a hammer disengage and locking sear as well as the trigger reset sear. from your post. it doesnt tell us what sear you replaced.
so after reading how your pistol vents gas as if it's an airsoft musket with the hammer not cocking back after the shot, it points mainly at the assembly part.
ruling out the catastrophic failure but that would just jam up your whole slide, which you've explained briefly it still somewhat, fires. and you're able to at least chamber the first round.
although your floating valve's spring MAY have got stuck or collapsed completely. therefore not generating any or at all, blowback power to rack the slide, in turn, resetting the hammer that will stop the mag from releasing any gas. that wont be from a collapsed blowback chamber or piston head since the recoil will still be sufficient to power the hammer lock.
more info is needed to help you out on that.
as for your 18c.
which brand is that? if it's the KSC, you actually need to cock the slide back maybe 1cm or so to 'change gear' due to the sears need that little room to lock into place.
otherwise forcing the select fire will cause selector sear damage and permanently damage your selector parts. (says in the manual too)
if it's a TM one, you dont need to do this step iirc.
reading it's a metal slide from factory settings, that rules out most of the irregular sizing for compatibility issue which i'm sure that's why you put that info in. it may just be the SA sear not reaching the hammer disconnect sear. take the slide off to see if you can see the sear moving when you go from neutral to engaging the semi auto
also, the other step would be manually cocking the hammer back, pressing all the sears down from the side of the hammer assembly
(reset trigger, hammer locked and ready to release. note: the hammer lock must be pressed down in some models before the hammer can get cocked back)
and pull the trigger, holding your finger on it... and re-cocking the hammer without pressing anything. now if it's your hammer assembly's fault. this should show in the next step and your hammer will still fall down without resetting the sear.
now with the hammer still dropped. reset your hammer lock sear by pressing it down. you should hear a click with that sear seating slightly lower.
cock that hammer and STILL WITH YOUR TRIGGER HELD BACK. the hammer should stay up.
if not, your hammer lock sear is either broken/worn/mis-aligned.
seeing how you can replace your own sear on the 19, hopefully you've just saved yourself 200hkd on labour in the shops
happy DIY!