Lord's Day does not mean Sunday. The only day Jesus ever claimed to be Lord of was the sabbath. Therefore, John must have been in the Spirit on the sabbath.
The verse in Colosians about not letting anyone judge us with regard to sabbaths is referring to specific ways people keep the sabbath, things they do or don't do on that day.
The law was not done away with when Jesus went to the cross.; it was fulfilled, or made complete. See Jesus' teachings in the Sermon on the Mount for a clear example of this.
Rather than seven churches or local corporate bodies, I think the seven churches refer more specifically to seven types of individual Christian, although the things written to the seven churches are applicable to local congregations as well.)
I used to think I knew the kind of Christian the church of Ephesus stood for, but after hearing Swaggart's teaching I am not so sure. Having said that, I think (or at least used to solidly think) Ephisus represents a Christian who is serving the Lord well but isn't as on fire and full of love as he was when he first got saved.
The church in Smyrna represents a Christian who is going through severe persecution, as many Christians around the world (including in North America and the rest of the West) are presently.
The church of Pergamos represents a Christian who has been serving God well. However, he works and possibly lives among very worldly and ungodly people. He is starting to buckle after constantly witnessing the filthy way in which the people he has to associate with on a daily basis live. He has started to tolerate certain sinful practices and has begun to view them as acceptible.