Well folks ... I think I FINALLY have a conclusive find!!!!!!!!!!!
Not sure if it was clever or lucky, but I replaced the O-rings with a rub of grease, but swapping the top and bottom over. Now, having rigged it all up again, making sure everything is nice and tight, I can clearly witness beads of water leaking from the chrome screw on dome at the base of the faucet (roughly a bead runs down every 4-5 seconds) .. so it is leaking and finding a way out on the front and back of that screw on dome, no matter if it's hand tight or wrench tightened.
So... would it be a clear deduction, that swapping over the O-rings is now showing me the failing O-ring from above, rather than water finding it's way down inside the base of the faucet previously, then seeping down into the cabinet below??
Either way, It looks like it's time to take the O-rings walkabout and "hopefully" find the exact replacement parts for this faucet model. It always makes me nervous when certain suppliers use the term "generic" .. so I will do my best to avoid that.
Anyway, thanks for everyone's quality feedback - I feel that I can finally move forward on this saga, although the ultimate reward will be to come back here and report NO LEAK!!!! *fingers crossed*
All the best,