normal map help with mirroring

I've spent close to a couple months working on this portfolio piece:
http://www.polycount.com/forum/showthread.php?t=132206
My first big question is about the "red errors" I'm getting on the mirrored islands. It seems like the parts around the bottom right that I welded together (so I could paint both sides) didn't bake right. I tried splitting them apart but it didn't help. I don't know why this happened.

I'm using Maya 2014 (usually I use 3ds Max, I'm resume building here) and XNormal so far. I decided to spent time simply studying/researching the matter. I've been deep into researching normals, smoothing groups, differences between bakers, etc for the last three days. Among my studies have been the follow in case you are interested:

http://www.polycount.com/forum/showthread.php?t=81116
http://www.polycount.com/forum/showthread.php?t=67479
http://www.polycount.com/forum/showthread.php?t=65790
http://www.polycount.com/forum/showthread.php?t=81154
http://www.polycount.com/forum/showthread.php?t=41776
http://www.poopinmymouth.com/tutorial/normal_workflow.htm
http://www.polycount.com/forum/showthread.php?t=68173
http://www.polycount.com/forum/showthread.php?t=107196

For now I will simply ask if someone can define the following terms:

"tangent basis" (How the tangents work?), and "average, welded and broken cage" (I'm guessing this is XNormal specific? Is averaging the cage the same as saying averaging the normals?)

I've read them so many times without really knowing what they mean exactly. Thanks