Is it possible to assert with certainty that the hadith "all of them are in the Fire" is of definitive authenticity, and that the opinion considering this addition weak is very tenuous, or is there still room to regard it as conjectural in authenticity?

The hadith of separation has been authenticated by many imams of hadith and weakened by others. The correct view is to authenticate it. The issue of definitively affirming the authenticity of a hadith varies according to the accompanying circumstantial evidence and the one inferring. Certainty of its authenticity is only possible for those specialized in this science. As for the layman, he must ask the people of knowledge. A solitary report can be definitively judged as false, or likely false, or held in abeyance, or its truthfulness can be preponderant, or it can be definitively judged as true, all according to the evidence. The nature of evidence as presumptive (dhanni) or definitive (qat'i) is relative and varies according to the perception of the one inferring.