First off, we don’t even know if limbo exists, it is a theological theory started by St Augustine.
Second of all, while God may not exist within in time, we do. This has entirely to do with our wills and how they are. God is a being of unchangeable will, meaning that he is the same yesterday as he will be tomorrow. We however are beings of changeable will. I can be saved today, but not tomorrow do to my will.
Limbo was an answer to the question of unbaptized children who died. The Church has essentially buried the idea in the last Catechism of the Catholic Church. To paraphrase they say that Salvation is tied to baptism, and while we are bound to the Sacraments, God is not. We can only trust that God in his infinite wisdom will do the right thing. Note they don’t say what the right thing is, because we do not know.
Limbo was never part of revelation and was simply a guess to fill a gap.
Edited, I forgot you second question. Mary was immaculately conceived because she was to be the new ark for the new covenant essentially. Nothing impure could touch the old covenant and so it was for the new covenant, Jesus. Immaculate conception was not for other people.