What do we do if SETI is successful?

The decision to not respond should not be considered an option for the UN. They can get a week max to decide what to respond, but a response needs to be sent quick. Otherwise you can assume someone will take the choice away and respond anyway. That someone could be a nation not liking the UN discussions, or it could be a rogue scientists with access to the powerful radios. (I doubt most of us could respond if we wanted to - even if someone is willing to break all laws they are either protected by too much security or they are too expensive to afford - but I guarantee someone who works at such a facility is willing to risk responding if governments delay too long)

Even if the UN makes a respond expect someone else to send a different one at some point.

No individual is going to have the resources to respond to an alien signal unless it comes from Proxima Centari (very maybe) or not much further. No current earth broad would be easily recognizable from Proxima Centari with earth technology - a factor to consider when thinking about why entities aren't being easily detected. A powerful and very carefully aimed laser might work for greater distances but that wouldn't be something that can assembled in someone's garage.

But oppositely, if naturally defusing radio waves could be somehow detected from some further away location, the aliens would know already we're here and indeed lots about us so hand wringing about responding seems dumb there too.