Troll Bridge

No. Troll Bridge is a short story of approximately 7,000 words, first published in After the King: Stories in Honor of J.R.R. Tolkien, edited by Martin H. Greenberg, in 1992. It was later included in Pratchett's short fiction collection A Blink of the Screen.

Cohen the Barbarian is a recurring character in the Discworld series — a parody of sword-and-sorcery heroes like Conan, now very old but still indestructible. He appears in The Light Fantastic, Interesting Times, and The Last Hero, among others.

Yes — a fan film adaptation was crowdfunded in 2016 and released in 2018. It was produced with the support of Rhianna Pratchett and is notable for being one of the most successful Pratchett fan productions.

No. The story works independently. Familiarity with Cohen adds depth, but the emotional content — aging, obsolescence, and the end of an era — is immediately legible without prior context.