They've got what they need for a Donna return if they want it - the computer at the Library. All they need do is say that a backup of her memories was kept there. So if she starts losing it the Doctor can go there again (going to the mainframe on the moon rather than the library itself to avoid problems with the natives) and zap her mind with those memories, which do not include the bit where she was briefly a Time Lord. No problem.
I doubt that the ring is significant, although I did at one point wonder if she and it were components of the Key to Time, a Time Lord gadget of infinite power that was split into sections and turned into a variety of objects including a woman in season 18(?) of old Who.
