As far as the mystery gear, I can't tell you why Eaton says not to use it...

But the main difference between the RTLO 13 and 18 speeds is in the Aux box. The 13 speed uses only a bushing in the mainshaft of the aux box on the splitter output. The 18 uses an actual bearing inside of it, because of the increased number of gear splits. You can change knobs on a 13 and get it to function like an 18, but you'll lock up the back box eventually.