... it's still the tstat housing. I can see a tiny stream slowly appear underneath it (different spot than before), and can hear a faint sound of air escaping. I pumped it to 9 psi and in a half-hour the needle dropped about 2 mm, if that, so it's a tiny leak. There's an awful lot of coolant all down the block, though, that doesn't look like it's come from the housing. What a mess.
I tried tightening the bolts a little more but I don't want to overtorque them (still have not been able to find specs). I guess since it's such a tiny leak, I should try it one more time, with water pump gasket sealant. I did not use it this round.
How do I turn such small projects into epic sagas ... .

__________________
1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)