Tim,
#1) I was told by my dealer that the tail lamp harness was NOT available separately.
#2) Yes but even that is an expensive option.
#3) Not sure since I didn't go that route. But I don't think you really need to pull the tail lamp assembly, you should just be able to pull out the harness because it is separable from the tail lamp assembly.
MY New Thoughts) A mechanic friend of mine said he would have simply replaced the sockets with generic aftermarket sockets. IE build a new custom harness out of commonly available sockets and connectors, not OEM parts. He estimated he would have done it for a 1 hr charge and less than $20 in materials. If I had it to do-over I would go that route.
Regards, Atlanta SRT6