so far some say yes it is a requirement of every believer, some say no regarding keeping the saturday Sabbath and feast days,
reply from the MJ forum here.
In general (I cannot speak for the whole):
MJ believes, as most, that we are saved by the grace of God through faith in Yeshua and His finished work on the cross. We do not keep the Shabbat to be saved, nor do we obey God's commands (as found in the Torah and Yeshua's words) to be saved. Rather we keep the Shabbat and obey the commands that we can (as best we can) because we have been saved to do these things (work out your salveation, etc.).
Just because Yeshua lived a perfect sinless life, obeyed Torah perfectly, doesn't mean that we no longer have to obey those same commands that He obeyed. Sure, we're not going to do it to the degree that He did, but does that mean we shouldn't do the best we can?
As far as the differences between SDA's observance of the Sabbath and MJ - SDA pretty much treat their Sabbath as mainstream treats Sunday - a few hours on that day set aside to go to church service and maybe Sunday School. Other than those few hours, the day is pretty much like any other day (except many do not work at their daily jobs on Saturday).
MJs begin their Shabbat at sundown on Friday and observe Shabbes until sundown on Saturday. During this time (again, in general) we do not work at our daily jobs, we don't cook, we don't do normal housework/yardwork, we don't handle money or do business (i.e., shop, eat out at a restuarant, etc.); those who are able to not use a car, don't.
Do we lose our salvation if we decide to go to Disney World on Shabbes? No, we don't, nor do we receive salvation for not going. It's a heart thing - what are our motives for keeping or not keeping Hashem's Shabbat. That's the whole thing about observing Torah - what are each person's motives?
What is required? A pure heart.
I know I didn't address all of your questions, but hopefully this will help somewhat.