[v8volvo]

Just read all this. Sorry I didn't comment earlier. Steve mentioned the thing I was going to say, which is that the VC studs can get screwed part way out and then their shoulder prevents the VC gasket from seating all the way down against the head. The other thing that can happen is that the valve cover's sealing surface can become warped, which becomes especially likely when the above situation involving incompletely-installed VC studs happens and the cover experiences uneven pressures and positions.

These engines rarely leak from anywhere other than the valve cover. I can't tell from all the info here if you have pinpointed exactly from whence the oil is coming. Is it trickling down from above? Getting flung up from below? You've now replaced everything in the T-belt area at least once and it's still leaking... spending a little more investigative time trying to figure out the source might be a time-saver in the end, rather than replacing more parts.

If the valve cover is straight, you can torque the nuts quite tight when you use the neoprene gasket.