What is the ruling on attending a mixed business dinner where there are people drinking alcohol, knowing that I sat far away from those drinking, and is there any sin upon me for that?

You have done well by regretting what has passed, for remorse is repentance. And part of repentance is the firm resolve not to return to the sin. And whoever repents, Allah will accept his repentance. The evidence from the Quran is: "And when you see those who engage in [offensive] discourse about Our verses, then turn away from them until they enter into another conversation." And from the Sunnah: "Whoever believes in Allah and the Last Day should not sit at a table where alcoholic beverages are circulated."