Although there are exceptions, ordinarily, Catholic discipline does not allow for the reception of Orthodox Communion. Orthodox discipline also does not allow for the reception of its Communion by Catholics. So, no, you should not receive Communion at a divine liturgy in the Greek Orthodox church. Actually, from what I understand, Communion is not usually offered during Greek Orthodox wedding liturgies, so it doesn’t appear that you’ll have that problem anyway.
The Sunday obligation is a separate matter. On Sundays and holy days of obligation, Catholics are obliged to assist at Mass “wherever Mass is celebrated in a Catholic rite either on a holy day itself or on the evening of the previous day” (canon 1248 §1, Code of Canon Law). Since the Orthodox churches cannot be considered a Catholic rite, the liturgies of the Orthodox churches do not satisfy a Catholic’s Sunday/holy day obligation. In short, you’ll have to go to Mass the next day on Sunday.
