The status ig the island is Reserva Parcial, or partial reserve so you should be able to camp legally.
I don't know whether the Ranger camp is still functional, but I camped there over ten years ago.
The fee should be MT400 entrance and MT400 per day camping.
A complication could be that Starlight Cruises have built an entertainment center on the island and I am not sure what agreement they may have with the authorities regarding access by the public to the island.
I would contact all the lodged on Inhaca and ask them. Look up Ross Ramos Camp, Manico Camp, Kanimambo Inhaca and
Zena Mar Lodge
Nhoxani Beach Lodge
And ask whether they have any info about Portuguese Island.