Sonoma 4.3 Vortec Intake/Head Gasket Replacement

[rharris]

2001 Sonoma 4.3 with 149k. We have had the truck since 2018 @ 146k. The truck always seemed underpowered and over thirsty. Anyway, two months ago and two days after passing his drivers test, my sons Sonoma heat quit. I reverse flushed the heater core and it cleaned out some black gunk and heat would work. Repeated this every other day for about a month. Replaced thermostat a few times, ran flush through multiple times, replaced overflow tank due to crud baked in the bottom, and could not get the gunk off the radiator. You could stick your finger in and get sludge off radiator. I assumed the radiator was gunked up from the last 20 years. I replaced the radiator and had heat for a week, and then the tstat blocked up and heat quit. New radiator looks like its on its way to the old radiator. After reading, it sounds like the intake gasket could be the culprit. The oil looks normal. the coolant is crudded up again. The 2 plugs i pulled are clean and dry. Picture below is the new radiator with some crud scraped off the plastic to give an idea of what the contamination is. The side mark is a marker. A few questions -

1 - The sludge in the coolant does not seem to be oil, and appears to be carbon-like. Does this support the intake gasket failure?

2 - why hasn't the coolant contaminated the oil if there is a leak?

3 - Are these engines as prone to head gasket failure as intake? I have the gaskets for the heads as well, just not sure if I want to pull them off. Gasket kit is Felpro

4- GM repair manual says to use angle settings for head/intake bolt torque (have new bolts). Is this crucial, or is a ft-lb torque wrench sufficient.

5- Any other suggestions on the problem before I go tearing it apart?

6- Is Dexcool necessary in GM?