I think there's been a gap of several weeks between the last episode and this one, so both Justin and Brian had some time to recover from the Pink Posse and there's no reason not to be happy now that Kinnetik is kicking off. I agree with arcady that Justin is proud of Brian as well as free of his demons.
As for Lindsay being on that stage, considering that she was the one who organized the benefit and handed over the cheque I figure she's representing the support from the community, like Cynthia is representing the professional support and Justin the personal. And Brian took her up with him first because she happened to stand next to him when it was time for the speech.