Is't harder to apply Maybank 2 Platinum Card for first time CC holder although I meet basic requirement? Or have to go through Maybank 2 Gold Card first?

* Got reject for the application of Maybank 2 Platinum Card. 

the banks always look up your other commitments， i.e. loans repayments , to see your payment behaviour in order to judge your reliability to qualify for credit card.

If you don't have commitment, probably can try with the easiest card with the least income requirement, then build your credit history from there.
but no harm trying with gold card, too.

if you have a good payment record and CCRIS rating is good, every bank welcomes you.

550-600 is pretty below Average, no? Mine is 750+

i think is below average

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Most airlines online payment gateway is in the US, so they all follow US time zone. This also applies to Malaysia Airlines, Singapore Airlines, Japan Airlines, EVA Air etc online booking. If you want to book a Malaysia Airlines ticket online on a Saturday with your MBB2 AMEX and get 5% cash back, always buy your ticket after 2pm on Saturday Malaysian time. But to be absolutely safe, buy it after 6pm on Saturday.
Tell them you want to apply for a credit card by pledging fixed deposit. You can't withdraw the fixed deposit as long as your credit card is active. This is what we call a secured credit card by fixed deposit.

Pledging FD is a guarantee basically bank is not offering you any limit. Well, once you uplift, your card will be cancelled too. So, in a way you will need to keep the FD for your card to stay active.
Yes, you will earn interest based on board rate.
Any benefit using that AmEx?

Better calculate properly whether FD is worth it. You're tying up (let's say) 30k of your money with the bank in order to get a credit card which max cashback is RM50x12 per year if fully utilize (ignore TP for easier calculation). RM600 total.

The difference between interest rate for the FD you're tying up (board rate) and promotional FD rates may be about 1+%. At 1%, that means it 'costs' you 300 ringgit per year in lost interest. Not to mention the lack of flexibility because you tie up your liquidity there.

Effectively you're halving your cashback in the best case scenario (fully utilize RM1000 on the dot every month). Once you count it as 2.5% cashback not so attractive anymore. Can still be worth it, especially if you're targeting categories that no other card can give cashback, but definitely needs thinking.