So I cleaned everything, with extra care for the stud threads and nuts. Roughened up the side of the washers facing the heads to avoid the bearing effect. Oiled the nuts and threads and put back the head I've removed reusing the gasket. Torqued down in three steps, waiting one hour between the steps. Everything by the book.
Filled up with distilled water and pressurized the cylinders. Same symptoms, air bubbles coming up in the thermostat housing. Already around at 1.5 bars.
Got new head gaskets from a local manufacturer. I know other Ghibli owners here using this gasket so it's tested. Made extra care to perfectly align the fire rings, for this I had to adjust the cutout around the dowels. But even the aftermarket one I've received from Italy had alignment imperfections. Having the edge of the fire ring in the combustion chamber can cause detonation issues.
Same process again. Same results... it has to be something else than the gaskets.
Strangely on the first gasket the fire rings are not crushed identically. Although I don't know their original measures, but right now the middle one is around 1.8mm, the two sides 1.85mm on the intake side, 1.65mm on the intake side. On the original factory gaskets I removed it's consistently 1.65mm everywhere. This is all just the LH side, didn't touch the RH head yet.
I'll do the pressure test from the other side and try to peek in with a camera checking where the water comes in. For this I have to close up all the hoses somehow.
The head slid down the studs without feeling any interference, but what if indeed a sloppy resurfacing causes the studs hitting the side of the holes because of non orthogonal compared to the main plane. But if the head slid down, that final tiny movements should be allowed, shouldn't they?