Hilton T&C doesn't say anything that adult reservations is for 2 adults. It says members benefit(like breakfast) for diamond and gold is for 2 guests. And all other maldivian hilton prperties never have this policy. Milaidhoo made an huge mistake when they joined SLH. They should have allowed the max capacity for standard room for 2 adults only. Then they would have NOT encountered this problem. Customers can book for 2 adults. If anybody wants to add a 3rd guest, they will have to inquire the resort. Then Milaidhoo can explain about the 3rd guest charges. In this way, you can't complain.
This is how the other SLH Maldivian property, Huven Fushi is doing. The standard room capacity is for 3 people on their website. But you can only put up to 2 adults when you book through Hilton. They clearly they you have to contact hotel for extra guest charges.

3rd person's seaplane transfer, I understand you will have to pay. But for the room charges, it is total bs. Its not just about Hilton, it is for every brand hotel's T&C. All registered guests are allowed to stay in the room. (meal plans are different). What Milaidhoo is doing is setting up a trap for point redemption clients. Nothing explained in advance. And give you a huge invoice right before you book the seaplane. What I don't understand here is that why are we obligated to purchase the meal plan for 3rd person? What if the 3rd person doesn't want to eat at the restaurant there?

For anybody who has a similar issue, I would suggest to bring this up to Hilton. Make sure your logic and rationale makes sense tho. It is their fault who allowed tot book under 3 adults with points. And you cannot even refuse their half board meal plan.