.. how does the coolant leak from the headgasket? How does it get there in the first place? ..

There are water and oil passageways between the head and block in the knockout areas of the gasket, some not that far from the cylinder wall, so when a gasket wants to fail, the narrow areas are often the easiest failure points.

Every time I see a head gasket discussion I think back to my wife's little old Mazda letting go back in the 1980s on her way to work. Before the end of our block, vapour started coming out the exhaust far more than normal. She had the smarts to pull over and stop. I decided to try limping it the five short blocks to a local service station figuring it wouldn't overheat enough to do head damage, plus it was only a crap little Mazda so I had minimal emotional attachment (if it had been my yellow peril 144GL I'd have been far more cautious). By the time I got to the station there was a mushroom cloud following me, cars were honking to get my attention and they all had their head lights on like they were driving in thick fog. One new head gasket later and the Mazda lived on to see better days, soon under a new owner after finding a nice '85 245 for her. 240 (and 140) exhaust systems are well known to collect condensation in the rear cross muffler with a lot of short trips (and why they rust out so prematurely), which makes for extra vapour when they do eventually get hot on a longer trip. It took her a while not to get nervous looking at wisps of vapour in the rear view mirror for 5-10 minutes going to work early on a cool/damp morning.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now