First of all, let me welcome you to our forum. As a former Protestant myself, I can definitely empathize with your questions. In fact, we have an entire forum dedicated to dialogue between Orthodox and Protestants. Many of these questions we who discovered the Church as an adult are still working toward answering, and your perspective would be most beneficial. For more complete answers, I invite you to check out the links Peter has provided above, but I will answer a few of these here.
1. In Protestantism, we believe in the age of accountability, which varies from person to person. What do the Orthodox believe happens when an infant or mentally disabled person dies?
We don't believe in an age of accountability as it's defined by the Protestants. In fact, our children take communion from the day they enter the Church, and even in the womb through their mother's communion. We believe that all people are born in the image and likeness of God, and that this perfection is spoiled by sin. Those who have not sinned are still in the image and likeness of God our Saviour, from whom all perfection comes. Babies at first are not able to exercise free will, and thus cannot choose to sin. But as they grow, they gain the ability to use free will, and from then on must choose righteousness. So in a way, the age of the first sin is an age of accountability, and requires repentance. This age does vary from person to person, as you say, and is generally pretty young, but could be delayed as in the case of mental injury. At any rate, it is for God to receive our repentance, and only He knows who has truly repented of all their sins.
2. Is science compatible with Orthodoxy? For instance, there is a lot of evidence for evolution, and many Christians accept it.
Most definitely. There's still so much left to learn that we cannot explain. Neither science nor Orthodoxy will completely answer our questions about the world and our place in it, and neither will they together--but they will tell us some of what we want to know.
6. What do Orthodox Christians, in general, believe heaven will be like? Is it purely a spiritual realm? Do we reunite with family members, or meet no other form of life besides God? Does it have physical qualities?
I believe (as do many Orthodox) that heaven is a restoration of Eden. It will be both physically and spiritually perfect. Sickness, violence, death--all will be removed, and we will live as Adam and Eve did. I believe also that it is a spiritual transformation, and that it will very much resemble the Earth we know today, but without the violent weather, wars, nationalism, racism, and hatred of all kinds, an Earth where all people are one with each other and with God.
That said, there is no consensus in Orthodoxy. We simply do not know what it will look like. My opinion is just that, and although it is shared by many Orthodox, there are also a number of other theories. We do know that we will be with God eternally, and that all the evils we see in our world will be removed. But that is really all we know.
7. What are some things you believe to be wrong about the Roman Catholic Church?
8. What are some things you believe to be wrong about the various Protestant Churches?
Essentially they are the same to us, as the Protestants came from the Catholics. Much of our disagreement stems from the Pope's proclaiming universal jurisdiction and subsequent excommunication of our Bishops. We returned the favour, and we've never been together since. Many in Orthodoxy lament this separation, and see the Catholic Church as our long-lost sister, and the Protestants her estranged daughters. Yet all Orthodox would still insist that the Orthodox Church is by herself the one true Church, whole and inseparable.
If that sounds strange to you, welcome to the club. It's a concept that seems familiar enough to Catholics, but to us who were Protestant, it can be a real stumbling block. I'm going on five years trying to work it out, and it's something I've just had to take on faith, at least until it makes sense to me.
I hope those answers help, and please don't hesitate to start threads on any subject you're interested in. I've discovered that people find it easier to post specific answers when the topic is sufficiently limited. It helps too to read what has already been written, as we have had a good number of knowledgeable people on this site through the years. But if you can't find a thread with the specific answer you're looking for, just start one up. We'll be glad to help where we can, and at least speculate where we can't.
Again, welcome to the forum, and I hope your tenure here is a long and profitable one.