If you didn't know the rule forbidding you, then it was not a mortal sin. One of the requirements for a mortal sin is that it was committed knowingly.
You should avoid taking communion anywhere you do not agree 100% with their doctrine. Part of taking communion is saying "I believe in what this congregation believes; I am a part of this community." If you're not part of that community, don't take communion.
Of course! Though I find it hard to believe you can't find a single priest anywhere to hear your confession.
To pray to God for forgiveness? Not really.
To confess to a priest? Yes, but it's different denomination to denomination.
There are different Catholic and Orthodox rites, but the differences are cultural, not doctrinal. Don't worry about them.