Jim,
I believe we need to be careful to make a distinction. It is not, nor has it ever been the official or unofficial teaching of the Seventh-day Adventist Church that Sundaykeeping is the mark of the beast.
Our teaching is that the mark of the beast is the persecution of Sabbath-keepers by Sunday-keepers. EGW and others have said that this will take the form of a national Sunday law in the United States. I'm sure of it. Already, without any Sunday law, it is more difficult to compete in business, and impossible to gain employment in certain trades unless you break the Sabbath. Our buying and selling are already curtailed.
But you know very well that if it comes down to actual persecution by Sunday-keepers of Sabbath-keepers, the day of worship is not really what is in question. In fact, when the Pharisees persecuted Jesus about the Sabbath, the day of worship or how to keep it holy was only a pretext for a bigger issue.
The issue at stake in the mark of the beast is, "Who is in charge? Who has authority on earth to make moral law? The bottom line is, who are we loyal to, God or man?
The Greek word for mark in Revelation 14 is "charagma," an etching, or stamp, but Strong's dictionary says it is like a badge of servitude. One who bears the mark of the beast is in some way serving the beast, either by obedience to the beast, or by standing for the principles or ideology of the beast. Loyalty.
That people keep one day of the week or another is important only as it indicates where their loyalties are. That many Christians worship on Sunday doesn't mean that they are intentionally serving the beast or trying to defy God. To their minds, they are honoring the day of Christ's resurrection. But when that time comes, and people are moved to actually boycott and bankrupt, or later kill others over a day of worship, it is clear that they are serving something other than Jesus Christ. That is the point of the mark of the beast- not a day.