Fisrt engage an instructor or dive shop, plenty of them in town. Go to PADI website and locate a nearby PADI dive centre.
Then you will knowledge development (read book + video) on your own pace and then over a few days (doesn't need to be consecutive days) you can do your five confined water sessions in one of the available pools of the said dive shop, finally over a (long) weekend you can go to Tioman or the Terengganu islands to complete your four open water sessions and pass the test and you're certified open water diver.
As for credit card payments you may need to talk to the particular dive centre you'll be engaging with.

Define fairly.
For me fairly = Good quality but cost more than others.

if you are short of time on island, then you will do the theory and pool sessions in town, your cost will be higher.

e.g.
OW at Seamonkey PJ  RM 1,200
Island diving RM 400 (not inc transport, lodging, food)

the plus point of this is, you will have a more relaxing time on island, no stress and rush, and wont fall asleep on the manual at night

Just to let you know that the there is a dive show at the end of this month at PWTC so you might want to check out and get bargain prices.
Otherwise you might just as well rest it until next year when you have more leaves.
There's an old (?) Chinese saying that goes like, 'Time is like cleavage, you have it when you squeeze it'

Wow. Honestly might be quite dangerous to go there after open water. Currents are unpredictable there.

Owh did you read about the sunken boat that was going from lombok-komodo-lombok as well? interesting read.

Experience came with more dives. As open water certification primarily teaches you survival skills, you will need more time to build up experience from fun dives, which judging from your lack of free time to do fun dives, I'd probably give diving komodo a second thought. Diving at those sites require minimum advanced open water with drift specialty since the current is unpredictable.

Recently I read an article (via wechat in Chinese) of a Chinese diver with 20 tanks experience diving at Canyons, Puerto Galera, where they encountered with massive down current.

Here are excerpts of the images of the dive comp only, if you can / want to read the article (in Chinese) click here (not sure how long the link will work).

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Theres September. Got a 3d2n long weekend. You could do a KL base pool than island certifying dive kind of dive.

September got 2 long weekends back to back...3 days leave = 9 days off 
But I have two non-diving trips planned in September, probably will get a day of compressed air fix during my stopover at KK.