After reviewing the video, it's clear that Garnett knew it was Rush's arm that he was grabbing. At first, Rush was in front of Garnett, guiding him away from the altercation. Garnett went back toward the ruckus, and the only reason Rush was behind Garnett was because Garnett pushed past him. I'm a big fan of the Celtics and Garnett, but he intentionally made physical contact with Rush. He grabbed Rush's arm. The rule doesn't have an exception for cases, like this one, where the ref makes contact first. Rush could have immediately given Garnett another T and ejected him. But he didn't. Must be a part of that pro-Celtics conspiracy masterminded by Stern.
As for Perk, he took one step across the sideline, but that was as far as he went. He did not proceed toward the altercation, and he never left the bench area. I don't know what the standard is; it's not in the rulebook. If the criterion is whether he crossed the sideline, he's in trouble, but the sideline is not generally a hard-and-fast boundary when play is suspended.