I too have come across this when engaging with Mormons, the ‘get out clause’ is of course known off pat by all missionaries.
“A prophet’s words are scripture, but only if he is speaking as ‘THE Prophet’ at the time. If he is just speaking as A Man then his words have no more authority than anyone else.”

Unfortunately there is no requirement for a Prophet to declare when and indeed if he is speaking as Prophet or Man, until his councillors and the quorum of the twelve had time to review his words afterward and decide in what capacity he was speaking.

This is how many of the more embarrassing declarations of the past prophets have written off and even been removed from the D&C after it was revealed to the later authority that the past prophet was only speaking or writing as a man.

The classic example is when JSjnr. declared the word of wisdom was advice and was ‘not by commandment or constraint, but a word of wisdom,’ (even though he was quoting a conversation with God) he was speaking as a man only, but when a later prophet (Brigham Young) declared the WoW to be a commandment, the ignoring of which can result in a member being barred from the temple and callings, HE was speaking as a prophet.

Firstly the Mormon concept of what is true does not neccesarily just rest with the scriptures, according to Ezra taft bensons 14 fundimentals talk anything that the prophet says is scripture, also these quotes i have provided are from lds.org the official site of the church that i would imagine most lds members consider reliable.

Secondly this website is about reaching mormons not mormonism as such, so what this means is if mormons believe this idea of the sacrament then that is what i want to engage with, even if some members like yourselfo do not accept this statement, which to be fair you have not said yet.