The GOA interfaith marriage website is also probably pretty useful for these questions:
http://www.goarch.org/archdiocese/departments/marriage/interfaithI'll take a stab based on my understanding, as I'm having my own crowing this Saturday and have been reading a lot about it....
1. Validity in marriage is the union of two Christians baptized in the Trinity, but not necessarily from the same communion of faith. That is distinct from "sacramentality" in marriage, which is the union of two Christians sharing a communion of faith sealed by the Eucharist. The crux of this "sacramentality" is a sharing of Eucharistic communion.
To look at it from another angle, is attending a parish where this "sacramentality" is not available any lesser of a participation in Christ and Divine Grace? Some Eastern Catholics might prefer to attend an Orthodox church without receiving the Eucharist than a Roman Catholic church where it would be available.
2. Absolutely.
3. Fr. Meyendorff points out that the Orthodox practice of separating the Crowning from the Eucharist has muddled this issue, as a "mixed" Orthodox ceremony can include crowns and a common cup, but avoid the uncomfortable issue of Eucharistic communion (specifically the lack thereof). He notes that if the Eucharist was restored as essential to the sealing of marriage in the Church (as it was even before crowns were used), the question you pose would again become academic.
4. I do not believe that a Byzantine priest (Orthodox or EC) would (or should, at least) marry one of his flock to an unbaptized partner. The only way for a Byzantine marriage ceremony to work for a "valid" but not "sacramental" couple is to forgo the Eucharist.
5. As I understand it, Orthodox churches do see marriages of Orthodox to other Christians as valid, yet not sacramental in the sense of being Eucharistic. That many fully Orthodox crownings are also not Eucharistic is a problem, as Fr. Meyendorff notes. In answer to your question he notes that, "....the Church, in each of such cases, hopes that religious unity of the family will eventually be restored and that one day both partners will be united in Orthodoxy." p.53 Marriage, an Orthodox Perspective.
6. Orthodox marrying non-Orthodox Christians in the faith of the Trinity are not excommunicating themselves, but are hopefully warned of the difficulty that the lack of Eucharistic communion in their relationship will entail and are counseled in the navigation of it. Should such Orthodox decide to commune outside of the Orthodox Church with their non-Orthodox partner, they would be removing themselves from the Orthodox Church of their own volition.
Orthodox who marry pagans wouldn't really be Orthodox in any practical sense of the manner in the first place, would they? At best, such a decision would be an indication of a severe concern of the disposition of said Orthodox, and any priest worth his salt would have much reservation about communing without first addressing these issues.
I could be totally off, but that is how it looks to me.