It's more that everyone in the group, due to their proximity, melted as one entire unit. So if you're within contact of someone touched by the Decay quirk, it'll spread to you as well. So less multitarget and more proximity spreading.
They've explained that while he can control every sort of fiber, jean material is the easiest. I'd guess muscle fibers are the hardest since first of all they're not even visible so it may be harder to get a grasp on them mentally, and they're so tightly woven throughout the body that he can't really tug on them. But yeah Best Jeanist is legitimately strong and I love it. Also yeah, new Vigilantes, I wasn't sure about it yesterday thus why I didn't post about it. It's a good one!