no, not fixed. you can call to change it if you want to.

why not able to register online banking?

first time user? have you get your tpin from mail? have you set up your phone at hlb atm?

Why you want to spend more than rm1000 or lesser than rm990 on this card where it only gives you 8.25% of cash back and no other benefits?
Just say if you only spend rm500, that's only 5% of cash back.
If you spend rm1500, cash back also same at 5%.
And with rm210 of AF and GST that you must pay, assuming if you always spend rm500 a month(that's rm6000 a year), you are getting rm600 of cash back.
After paying the fees, that's only 3.25%.

Since the cash back is credit in on the 28th and posted by the 29th every month, does the credit limit increase by 28th or 29th? I have maxed out my credit limit at the moment to ensure the GST won't be charged on Feb, even though I doubt that it is possible

due to technical error, i think they will revise it.
for new member only will receive one time rm50 cash rebate.

you may call the cs to confirm it. but i think you don't have to since you are earning rm100 from this error. lol...

You spent close to RM800 in one single transaction. Not so "WISE" already. Because of this transaction, you will need to spend min RM1200 instead of RM1000 for Feb CB entitlement.
Parenthood superstore is under which category ? Under your chosen category ?   
I guess you only have one credit card ?

I'm try to register the maxis online pay, and will it charge RM1 to out credit card?

Wait for their statement don't know need to wait how long. They have been experience technical error and cannot generate statement since few weeks ago..

As you said, if.
But you need to check on your statement date also.