Of course people will receive the mark of the beast of their own free will. I would certainly not limit that to those who worship on Sunday instead of Saturday.
1A) The whole business of Sunday being the mark of the Beast is such weak sauce that Adventists should be ashamed of that claim, if "being Biblical" is actually a real thing they actually care about. I say this because of all the 8th Day Typologies, Rituals etc. in OT Judaism and their prophetic significance.
1B) Not to mention the entire history of worship in Judaism leading up to the New Testament. e.g.- Purim and Hannukah were established as official holidays without an official divine decree. The same goes for synagogue worship and it taking place on the sabbath, that came from tradition and not from scripture. The entire synagogue system was invented off camera, to speak, during the Babylonian exile as a stop gap to give the Jews a way to worship since they could not go to the temple.
2) Besides this there is the trail of Doctrinal Development of the Millerite and post Millerite believers, which will bring you to the "Shut Door" and all the problems it caused with basically invalidating the prophetic status of Ellen White and the other Adventist "Pioneers" who were having the various early visions that were the source of these doctrines.
3A )Besides, this there is the whole unwieldy nature of "Present Truth". You basically are changing the rules of the game after maybe 3/4 of it has already been played under a different set of rules etc. There are really big, big problems of this when it comes to a number of scriptures and doctrinal topics on a number of different fronts of epistemology, God's nature, the nature of the Gospel, or even how the Church is supposed to function as one body.
3B) Common sense should warn people this is untenable. It is like thinking you can change directions of a enormous vehicle like an aircraft carrier on a dime, or expecting the tail to wag the dog. The Christian Church is a world wide body that stretches not just the entire world, but across 2000 years of history, and into higher dimensions and the future.... (But maybe that is the problem, because I can think of other unique SDA doctrines like Soul Death that make matters worse. Because Adventists do not believe believers have a spirit so to speak).
4) Finally, Adventist are not the only believers that have "The Spirit of Prophecy" as a modern thing. In the 1990s, I was very active in the charismatic movement and the "Prophetic Movement" that was going on with that. I saw and experienced some good things, but actually saw many problems too. I actually think, if Adventists had more of my experience, they would be much less willing to give Ellen White the rubber stamp. I have read various statements of Adventists especially concerning the "Spirit of Prophecy" talk about "judging prophecy", but it does not look to me that they have ever really done that, only paid lip service to the notion, because they know that skeptics and critics will bring up this topic at some point.