Are all Hadiths subject to scholarly disagreement regarding their authentication and weakening, and if so, how can one trust them, or are there Hadiths upon whose authenticity there is a consensus?

The hadith (definition) are not all of the same standing. Some are agreed upon as authentic, while others have been debated regarding their authenticity and weakness due to scholars differing on the conditions of the narrators, the chains of transmission, supporting evidence, and corroborating narrations, or due to their differing on the principles of al-Jarh wal-Ta'deel (critique and authentication of narrators). If the masters of this art agree on the weakness of a hadith, then the authentication of it by later scholars is usually incorrect. As for those who are not proficient in this science, they can follow a trustworthy scholar, and the early scholars are more deserving of being followed. What they deemed good or authentic is trusted to be established, and what they did not deem so is considered among the highest types of weak narrations.