Re-reading your post, I do not agree with the encyclopedia. I always understood that Limbo was eternal. I don't understand how they can say otherwise.
Moses and Abraham did not spend eternity in Limbo, yet, they were never baptized.
When I took CCD classes, I assumed that Purgatory was eternal. But there are only four Last Things: death, judgment, Heaven and hell.
The solution to the problem of infants dying before receiving baptism that makes the most sense to me is that God would give these children a chance to choose for or against Him before death. This would not negate the necessity of Baptism nor would it conflict with His ordaining will, expressed in Scripture, that all men be saved.