The compression numbers look OK to me.
Did you have the rad cap off during your leak down test? And if you did, there were no bubbles?
I don't know much about leak down testing, but it seems a bit complicated and maybe prone to set up and procedure mistakes.
Did you use a 2 gauge set up?
You are getting pretty good compression from an engine with abnormally high leak down values, and I tend to believe the compression numbers over the other. Also, exactly 52% on all but 1 cylinder seems suspicious.
On the old topic, that puff of white smoke on start up and during extreme deceleration is in fact the valve seals, but that is simply an annoyance and is not connected with overheating and other reported issues.
If you had a head gasket failure, the leak down pressure test would have injected air into your coolant and bubbles would have shown up in the rad.
The explanation for that condition being tied to overheating is, the combustion gases are extremely hot and direct injection into the coolant quickly overcomes the radiator's ability to cool the coolant.
Since you are in an experimental mood and have resigned yourself to removing the engine to perform a cure if necessary, anyways, I would try one last thing. This is based on the idea that if it is a gasket failure, it must be small for it to take a half hour to add the unwanted heat.
I would get a quality "stop leak" product (GM got very good at formulating this stuff) and follow the instructions closely. Anything that has solids should be pre-heated before installation.
That will go a long way in deciding if gaskets are an issue.
I expect to hear howls of protest for anything this primitive.
However, I bought a used SHO with a horrible water pump leak (one eighths cup every minuted) that was doctored -up to pass inspection with this stuff. I operated it this way for a few weeks and the heater rad never gave me problems like reduced efficiency. I only discovered the monster leak when I changed the coolant in a maintenance routine. When I changed the water pump, there were no lingering issues associated with the stop leak product.
And you can back flush everything after the fact, if you have concerns (which is what I would do too).
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