The social message of the play is shown by the physical representation of the Birlings" household and contrasted with reports of the desperate attempts of the workers to increase their poor wages and drab lives; lives forced upon them by the Crofts and Birlings of this world.
The Inspector champions the cause of the poor - in the form of Eva - against the rich - walking into the lion's den to do so. The Birlings and Crofts have wealth and power - the inspector assumes the mantle of the authority of the police in order to exert his own power - although he is lying, his power is no less real.
Are Eva and Daisy the same person? Is the photo that Goole shows everyone of the same person? Does it matter in the end? What we learn of her contrasts sharply with what we see of the Birlings and Gerald. We learn that she was kind and gentle, was genuinely fond of Gerald, though knew that their relationship had no future; was honest - refusing to accept stolen money from Eric, but did not give him away to his father or report him to the police, either for the rape or the later theft. Birling's reluctance to report Eric has much more to do with his own reputation and the hoped-for knighthood, than any wish to protect his son.
Eva/Daisy is a symbol - it doesn't matter whether the story is true or not. She stands for all the people we meet, every day - people we know nothing about but about whom we make assumptions based upon our own sets of prejudices - how a person dresses, speaks, the colour of their skin, the job they do or do not do, the music they listen to .add your own ideas. You assume certain things about me because I am a teacher and I speak in a "posh" accent. Almost everything you think about me is likely to be wrong, but it doesn't stop you from having an opinion about me. .
The Inspector uses Eva as a weapon in order to try to change the attitudes of the rich, privileged people at the dinner party.