Panama has full right to set its own tax rates. Developed countries have full right to deny Panama's corporations from entering their markets, and even forbid their own corporations from dealing with their Panama counterparts.

Or, more realistically, the developed countries will pull any of the dozens of levers to make non-cooperative countries' life miserable.

Yes, as long as Panama wants to continue doing business with the US. Guess what, most companies that are incorporated in a tax heaven do nothing there except tax and accounting, so they will pressure Panama to agree to the US' demands so they can keep enjoying their privileges.

It's not about rights, it's about ability. If Panama wants to trade with the US or it's vassals, it will do what the US says. China doesn't have to listen to the US, if Panama wants to defy Yellen, it can deal with China - although China may not see Panama as worth it, I don't think that tax breaks are an export that China is desperate for.

edit: think of it like this - you have the "right" to eat meat. But if I refuse to be friends with people who eat meat and you need my friendship, you can't eat meat.