Thanks. That sounds reasonable. A jolt hard enough to break the radiator mounting might(?) have done internal damage to the 5-year-old radiator. I'd consider that reasoning conslusive except for a patch of other odd behavior over the past 6 months that made me suspect that the head gasket might have become damaged:

The Mobil 1 looks (well, looked, before I changed it) a little muddy-ish, with no (or very little) gasoline smell. The engine's behaving itself, with good smoothness/power/mileage; the Check Engine light's not come back on for more than 3 months. But, over the fall/winter, it anomalously rang up a slew of codes: 543 454 414 455 542 (but I was unable to check them for months so I don't know if they appeared all at once or not; the biggest symptom -- except for one pretty big, sharp "cough" under acceleration (which may have been accompanied by a momentary exhaust plume) soon after an oil change -- was the CE light itself, which initially would only come [back] on during a long idle after it'd been running for a while). Until this oil level issue, I was gradually coming to the conclusion that that anomalous engine misfire/codes issue, whatever it was, was in the past. But now I'm unsure. I just now manually checked for more codes and the LED blinked 1-1-1.

Is it possible (say, as a quirk result of having possibly marginally overfilled the oil last Autumn) for a head gasket to become damaged in such a way that it is breached between the oil and coolant passages without breaching its compression/combustion sealing function? (Or can all channels be suddenly breached in such a way that they, or some of them, 'heal' over time?)

Does anyone know the limits/margins/risks of excessive oil level (it seems to have overfilled itself to a higher level 2 weeks ago, w/o syptoms, than it was marginally overfilled at last Autumn's oil change)?

I wonder if it makes sense to try to check for internal radiator damage w/o removing the radiator. (I know the radiator's finished, but if replacing it is not going to solve the problem, and I can know that beforehand, then I maybe have a bigger decision to make... I just found out that the best service advisor at S. Colorado's only Volvo dealer, who at times was my best friend there and at other times was my only friend there, is no longer there. So probably no compression check today...)