Very very interesting, and wow, you guys are upcoming debaters here!!
I completely agree on the point that Voldemort did often underestimate his enemy. Even during the time Harry and Dumbledore got the locket, he had overlooked the fact that a child could get through in the boat along with an adult, as he never expected a child to be able to across. Then again, when did Voldemort expect Dumbledore to be his enemy... Same goes for the ring, i think, as you rightly guessed!