I think your only option is to get the 7am ferry from Tongsala,the main ferry port and town in Phangan.
Lompraha fast ferries takes about 20-25 mins to get to Mae nam pier on samui then 15 mins or so to the airport.
Sadet is at least 20km from tongsala so you would have to be up pretty early to make it on time.
Haad rin Queen doesnt leave until 9.30 and takes a lot longer to get back to samui.
ferry timetables from phangan to Samui are:
Lomprayha 7.00 11.00 1600 and 17.30 20 mins
seatran 11.00 and 16.30 20+ mins
Haad Rin Queen 9.30 11.40 14,30 and17.30 50 mins
Songserm 7.00 45 mins
The ferries usualy leave on time but at that early hour not many Thais are on time!
Lompraha ticket will include bus trip to airport but you will have to wait for it to fill up and not everyone is in any sort of hurry to get of the ferry and loaded in the bus,plus stop offs on the way!
If you have been to Phangan before then you will understand that its not really a "tiny island" and seems even bigger when traveling on some roads

.
If you can I would go back to samui the day before on the late ferry then you can relax and have a swim after breakfast and get to the airport in plenty of time.
The flights in and out of samui dont seem to go hand in hand with the ferries to Phangan,this may be done on purpose so that people dont just go to phangan and not bother with samui?
Hope this helps!
Den