I use backslash to open the nerdtree. I don't want vim to pause for a moment after I hit the key, so I avoid having other mappings beginning with backslash. I figured other people might use backslash for other things.
Use <Plug> mapping as repeat command, not '\\\'.
Don't map '\\' or '\\\' if a mapping exists for a single '\'
First commit is a no-brainer. Accepted. As for the second, I think it might be a little confusing if a \ mapping blocks our \\ mapping, so I'm adding the explicit requirement that <Plug>Commentary be mapped elsewhere. I think that covers all the bases.