Tom, Hello neighbor, I'm in Enfield. Awesome post. Seriously, this is a very good post.
A coolant leak where you are describing it, can often be from the end cap on the LIM.
When ordering gaskets, ensure you get that gasket. It's GM part number 24505049 and called "Rear Int Coolant Cover".
Being a 2004 you do have the aluminum upper intake manifold and that should be resued. Only the plastic ones are known for having a problem in the EGR port area, yours will not. The UIM gasket should be reusable, check it for any marks or issue spots.
If that's Ultra Copper permatex, it's the only thing I suggest to people. One of the best you can get that is rated for oil, gas, water and coolant.
On the elbows, I would not suggest RTV. If there is a problem down the road, RTV is a pain to clean out of the coolant elbow holes. I personally like to lubricate the o-rings and that's all.
