btw, the cooling system no longer bubbles when the engine is cranked.

my plan for the leakdown test was to plug the cylinders not being tested then test for pressure through the oil dipstick

another thing i observed while the head was off: the inside of the INTAKE manifold was jet black.

Follow EricTheCarGuy's suggestions closely. I did not look at the whole video for a while...
The black intake manifold is likely caused by the EGR system. It could also be caused by oil and stuff coming in through the PCV. Maybe it is blow-by deposits.