Actually, I find this question worded properly.
"Do Seventh Day Adventists actually believe that someday their salvation will depend on what day they go to church?", Short answer is Yes.
Is "that someday" today, No.
Will the first/sun day people make first/sun day a salvific issue, in the future with the laws of men and governments, YES !!!
AV Re 13:15 And he had power to give life unto the image of the beast, that the image of the beast should both speak, and cause that as many as would not worship the image of the beast should be killed.
A future theocracy is not hard to spot at all. Are you ready for it's fulfillment ???
Just a note of comparison, in GOD's theocracy, had sabbath sins as a punishable sin with death.
Do you realize, how satan has counterfeits of GOD's authority ???
Yours in Christ, Michael