a) The best way to get to Octave hotel.
You'd have to take the ferry to reach this hotel as its in Male. The airport is on an adjoining island.

d) Which ferry to take from airport to Male city
You shouldn't have a problem finding these as soon as your out of the airport

c) Any taxi from ferry ( Male) to Octave hotel. Does taxi runs late at night. is it safe to take a taxi at this time of night . What is the fee and any surcharge for taking a taxi at night
Not sure about this, as i've walked around on Male for the most part

d) is there any phone booth at airport to contact hotel.
I vaguely remember seeing one once you've cleared customs. In case you can't find it and need to make a call, head to the resort booth where your main booking is and am sure they'd help you out. Most of the major resorts have representation at the airport in the form of small booths, where's there's a rep.

e) How to go back from hotel to airport.
In the same way, ferry from Male back to the airport and this time i'm guessing you'd need to make your way to the sea plane terminal which is right next to the main airport terminal

g) Do I need to exchange US dollar at airport. Not sure which currency to use in Maldives. Can I pay a taxi drive/ hotel fee in US dollars
Yes you can exchange money at the airport itself. Exit customs & on the left you'll see an ATM and money changer.

i) Do I need to buy a simcard when landing at MLE airport.
Not sure about this.

Extra bits of info:
You could also consider staying at one of the hotels on Hulhumale. This is basically the island where the airport is and so it would save you the ferry ride. Prices are bound to be the same as what your paying for Octave. Let me know if you need some suggestions on some hotel options for hulhumale. Lot of people do this as a stop over/ transit stay when landing in late or if they have an early departure the following morning..