First off you'll want to download DevKitARM. Just follow the guide here and you'll be fine. From there iirc most of my projects include a compile.bat which works across most if not all my projects in the event that one of them doesn't include the compile.bat. If you need an example to test on, use this project and go from there.
Close, it is only 1X 01 00 00. Only the lower half (the right half, the part with the lowest value) is applicable. That is the palette slot in the palette viewer that the OW will load it's palette into and then use.
That is for a different issue. You litterally just need to hex edit the byte that controls where the palette is loaded to. That is in JPAN's manual, written out very clearly in the overworld section. Look for the picture of the bad player sprite.
My routine can be applied to a smaller image very easily. You just need to change where the data will get placed in the vram. Large images are the easiest because you don't have to worry about block wrapping. Also, my routine was based around the premise that you have been warped to a 1x1 all black map so that the vram is always locked in the same place. I could never get the calculations for where the current screen was located to work properly so I just forced it to be the same.
Well I can't see that, haha. It just looks like the crash screen I get when I fail to do +1. Now, my guess is that you overwrote the tilemap and now it is using the same tiles for every single block and those are not the right tiles.