Tear into it, or bolt in and go???

I'm not sure what you mean by clacking and clattering all over the place. Do you mean while he is cranking the motor over for a compression test?

Do you do your compression checks with the engine cold?

It should be immediately obvious on start up if you’ve got one or more lifters bled down. And compression checks are supposed to be done with the engine hot. So yeah, that should have been obvious.