Here scribe 2 of 𝓟 started to write “and he sat upon the judgment seat”, but after writing the preposition upon, he immediately crossed it out and wrote inline the correct preposition in. In all, there are four passages in the text that refer to “sitting in /upon the judgment seat”, but as David Calabro points out (personal communication), there is a clear difference. When the preposition is in, the text refers to a judge functioning in his office:
But when the text refers to the judge literally sitting in his judgment seat, the preposition is upon:
Thus the preposition in is undoubtedly correct in Alma 4:17.
Summary: Maintain in Alma 4:17 the immediately corrected reading in 𝓟: “and he sat in the judgment seat”.