Man, I really don't have much to offer.
That pump is pretty tight and so if there was extra pressure in the system from a bad head gasket I'm almost certain the degas bottle cap would fail first as it's rated to 16 psi.
I would have to pull that pump off again and double and triple check everything. Even put a straight edge on both the pump and block to make sure they are true. Then I'd check for any nicks or cuts in the block that may be allowing the coolant to seep past the rubber o-ring seal.
I once had an old Chevy C10 with a 350. I put a new Edelbrock intake manifold on it and the thing would leak oil at the back. Come to find out the intake was warped so I got another Edelbrock. Same damn thing! Oil leaking in the back. I returned the second one and got a Weiand intake instead. Had zero issues with it. And so maybe you got a bad pump? I used a Hitachi brand water pump on mine when I replaced it on my 2.3.