Your pressure test used a very low pressure pushing outwards from the coolant channels.

When in operation, the head is extremely hot, the pressure in the cylinder is >1000PSI, and the head bolts have it clamped in position. All these things add up to make leaking from the cylinder into the cooling system far more likely than the other way around.

Sometimes it pays heed to listen to what other people are telling you from experience. Again, it's your car, your time, your money, your effort. You do what you want.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)