Ok, first of all, read the full guide. The answer to line 2 is in there. Since tomorrow is a holiday (for me, at least) I'll be nice(r) here.
No, I have not successfully unrooted. Neither will I unroot, as I have a track record of never sending my phones for repairs, even when it has warranty. I simply take real good care of my phone. That said and done, unrooting is simply flashing the right RUU for your phone. The list of RUUs are long, but you should be able to identify the right one by comparing carrier names and the kernel version. Go into
Settings > "About phone" and list out the information inside, as well as your carrier name (unless you have an unbranded version)
Nandroid backups EVERYTHING. Titanium merely does the apps and their data. NANDROIDs will restore everything to the original condition, unless it is corrupted.
Radio here really refers to the kernel that is used to control you GSM radio. It will also, to a certain extent, affect your other hardware, like the processor. So, it is important to get the right version. Most ROMs will specify (kindly) the recommended version of the radio to be used. Simply download the right version and flash the radio and then the ROM. Some people have better experience with other radios than the recommended. I, for instance, use the latest radio (32.48.00.32U_5.11.05.14) which gives me better signals in my 12th floor office compared with others. Here, it is important to experiment with other radios if you think they give better performance.
Most of the information you need is in my guide, except that I link back the ROMs to their respective originators because they deserve at least the credit for the work that they have done. I also don't include the radio files because different ROMs recommend different radios and unless you know what you're doing, I'd recommend that you use the one that they say you should.