PAA: In theory. But such a person would have to be a Bar Hachi of disagreeing and be able to give Ra’ayos to defend it. Joe Schmoe can’t come off the street and say he wants to Pasken like a rejected opinion. R’ Shteinman can prove why a rejected opinion might not be K’firah. Shmuely Yanklowitz can’t. And certainly not in this case where he just asserts it and doesn’t even attempt to give a Torah backing to his opinion.
PAA and DY: “Emunah Sh’leimah” is just a phrase they put in the Siddur. The Chiyuvim (which differ slightly by each Ikkar because some Ikkarim lend themselves to absolute proof/knowledge more than others) are to believe they are true, whether or not we can contrive hypotheticals to the contrary.