I got the part and put it on/in, success there.
But I still have leaking from the item in the top photo in post #4 despite changing it's O-ring, wrapping it with pipe ramp and even using sealant around all that! Geez. I guess the plastic it screws onto must have a tiny crack somewhere and I cannot see it without taking all that apart around it.
It'll have to be a jug under it and that collected water albeit not much at all will be used to aid in flushing.