First off, if you want to root, I think you will be able to - your BL version is 4 (5th from end number in the build version or the baseband version). It's not free to root this way, but I believe you can. However, if you do that, then I don't think you can update to any newer version of stock ROM, because you cannot update the bootloader (which is part of the other releases). Also not, rooting may wipe your phone too.
As for the other question, show me your full software info page, so I can see the CSC codes and other things. I want to make sure they make sense.
My guess is that because you have a U phone (as opposed to U1 with a multi-CSC setup) that you would not be able to patch/update the phone. This is because the U is typically locked to the carrier updates, and if it's looking for USCellular, that would explain why your TMo carrier can't update it. You could reflash to a U1 ROM using ODIN, and this should allow updating in the future, but it will likely require a wipe (you could try an upgrade flash, but I don't recall if this is possible from U to U1). This will also update the bootloader which means you can't root today, but you would be up to date.
So, decide how you want to go first... because you can't have both (root + updated... unless you do custom roms, which I am not versed in, but you are liklely going to give up some things with custom roms, from the superficial reading I've done there)