I will give this a shot. There are a lot of little Gremlins that can and often pop up in these processes.
Question 1. I tried to reblue all the parts. I used Birchwood Casey cold blue. It came out spotty and splotchy all over the barrell. I had the metal nice and clean. What did I do wrong?
Birchwood Casey will get you to a good result when multiple coats are applied. Major issues are exceptional metal prep and clean surfaces to start with. "spotty" immediately says to me that there are surface contaminants. I clean, rinse and clean again. Always wear gloves (powderless is preferred) and your goal is to not touch the metal if at all godly possible with ANYTHING once ready to apply. I will say to evaluate every thing that came into contact with the barrel before application of the bluing.. Rags, solvent used, how oily it was before starting, anything on the gloves to cause cross contamination? Was your work area dirty from previous projects? or where you clean your guns?
Did you touch anything in the area that might have had anything on it? did you scratch your forehead with your gloves on then touch the barrel? those are the first questions I would ask. Did you use clean, new solvent or from an old can you had in the shop that could have had contamination in it?
I never use the same rags twice for any application,(wipe, toss, wipe, toss repeat) and if i even think I might have touched anything or if i question myself if i did or didn't, I replace gloves.. lots of times, I find it handy to put on 2 or 3 pairs on top of each other, then shed one off as I go if and when need be.
Ive seen issues where someone used starched, clean cotton rags, rinsed a part with acetone, and the starch and acetone mix was the culprit of the contamination after hours of testing.
Question 2. The bluing did not stick to or react to the reciever and trigger guard. Is this because it is possibly a different type of metal? Whats my options spray paint?
It shouldn't have, its a different metal and Birchwood wont work on it. Its aluminum and you will need to use Aluma hide or similar aluminum blackening agent.. those are also very tricky and the results can be hit or miss.. royal pain for me and my experiences and I most often paint or cerakote aluminum refinishing projects whenever possible. that way it will last forever and look better than factory. I have also had great luck with that ceramic header paint that you bake in the oven.. that's always an option and its 5$ a can at Auto Zone..
Question 3. If I take the big internal part apart. ( not sure of correct name, It is the one with numerous springs and little easy to loose parts) How difficult is it going to be to reassemble it?
I assume you're talking about the bolt. Not too difficult to strip, but watch out for all the little parts and document where they go. honestly, unless you are having function issues and parts need to be replaced for safety and or function, I would leave it alone, generally a good shot of brake cleaner then a soak in Hoppes #9 for an hour will get pretty much all the crud out of the little nooks and crannies with a light scrubbing with a toothbrush. Blow out with air, oil and reinstall..
Hope this helps, Its not that hard to do honestly, but all the details are in the prep work, the actual "job" is the easiest part to be honest.
Bad preparation kills most jobs before they start..