"I think though that many of us are uncomfortable with the 1854 definition placing the Immaculate Conception on the same theological plane as Christology, Triadology or Pneumatology."
Precise definitions can be troublesome. About two years ago I had a discussion of the Immaculate Conception/Original Sin with a Latin Rite deacon from England. His understanding of Original Sin and what may be meant by the IC was almost totally Orthodox. He pointed out that Aquinas himself rejected the idea, at least the idea as it developed into what the Pope proclaimed in 1854. Defining something like that seems assuredly to lead to contentions, which is why I agree that some sort of theologoumenon on the issue seems the better way to go. I can't help but think, however, that the IC is absolutely necessitated by the "traditional" Latin Rite concept of Original Sin. Its simply one of the many results of that Augustinian, or perhaps, distorted Augustinian view of the human condition.
>>>Precise definitions can be troublesome.
Indeed, the pronouncements of the Council of Trent, in certain cases are prime examples.