Yes Harald offered to be their slave personally. He didn’t sign up his nation.
Yup Harald is prepared to get on his knees and get to work if it means the Giants on Elbaf can have access to a better future and he believed this to be only possible if Elbaf joined the WG coalition and started mingling with regular folk. This is pretty similar to Otohime's approach on Fishman Island. It's a pretty naive way of thinking but it's very realistic unlike what edge lords on here woud like to believe and it actually beared some fruit even if he wasn't able to utlimately achieve his goals. His efforts in conjunction with the help of good marines like Garp is the reason why Giants have been able to somewhat integrate with the marines and seems to have greatly improved giant/human relations.
The reality is that the WG is such an immense force that even Newgate who was revered to as the World's Strongest Man didn't want to instigate it with 10 foot pole. The WG are the defacto rulers of the verse, you either challenge the status quo or you bend to it. Choosing to challenge it will only lead to death except you are Nika. Rocks tried and got swatted like a fly. Harald is going the other route. It's a pretty interesting contrast with Rocks and there is no correct method imo.