Regarding Marriott's IT systems randomly adding another person to my reservation, with the additional seaplane fee - I emailed the hotel and they responded back this morning:

"Further to your enquiry, please be advised that we recently moved to new systems and therefore the way the reservations details are displayed have changed which has caused some confusion. Allow us to clarify the charges for you as below:

Return seaplane transfer at USD 695.00 per adult x 2 adults = 1390
Mandatory Government Green Tax at USD 6.00 per person per day x 2 adults x 5 night = 60
Grand Total = USD 1,450.00

Please be advised that you will not be required to pay the extra person supplement rate which is currently displaying on your reservation and disregard any amount which is exceeding the grand total amount.

We hope that the above information clarifies the matter. As this is a system glitch I am afraid I will not be able to correct the amount which is currently displaying on your reservation. However we will not be charging any amount which is displaying as an extra person supplement."