Im awful at this stuff, but here are my two cents
1)Mary is important for both of us and it largely arose due to conflict with Nestorius. However we dont put too much importance on her, while Catholics have numerous stuff like Rosaries, calling her co-Redemptrix and so on.
2)Neither of us think that leaving the church is ok, as you are severing yourself from God (to whom you are organically connected with Eucharist). However, if you are baptized, technically, you always remain Orthodox (I think its same about Catholics). So lets say that you are Orthodox, then became Atheist, Muslim or whatever, and then go back to Orthodox, you dont get re-baptized, but instead go to confession and mention to the priest that you have strayed away. Even non-Orthodox baptisms, if done in the name of the Trinity, can be acknowledged because of Economia.