1. I use roll pins for pinning items together sometimes when alignment is important. I just design holes on each piece to be joined and the the roll pins do the hard work for me. Otherwise, a groove or something (V shaped, like you did) will work. It probably isn't worth the effort here but I have even printed jigs for parts I make over and over again. Then I use clamps or rubber bands to hold everything together while the glue sets.
2. I like using superglue (cyanocrylate, if that's the right spelling). The gel form is the easiest to work with for me. It holds well. I usually am gluing PLA together. But I have also used Loctite Go2 glue with success (again, on PLA). That stuff takes longer to set but still works well. I use it on bigger jobs.
3. The strength of sections depends more on the orientation of the layer lines in my experience. 0.3 inches should be plenty for this application, regardless of layer lines.
4. I am also at a loss for paint. I would recommend checking out Youtube. There is a community there that makes costume props with 3D printers and they have to paint their creations.
Have you printed anything with the Ender yet? I got a Creality printer about 3 years ago. It's a beast! But it didn't start out that way. It was kind of a pile of crap until I got the bed leveled and printer settings dialed in. I'm not sure if you got through that part or if it's needed on new machines but I would encourage to take deep breaths and work through it. It's worth it. The power to think up something and then make it poof into existence is really fun!
I'm also curious to see how your bricks come out. I couldn't find the video I saw where a guy said printed bricks usually don't come out very well. So he printed a mold and then cast them in resin. It was pretty slick.
Keep us posted!
