Dear,

1. Does your GROSS income RM5000 include allowance? or allowance haven't add into the gross income

1. Based on the details given by you, Your max loan eligibility for each bank is as follow: 
  Rm
AMBANK  337,892.86
HONG LEONG  552,788.69
MAYBANK  423,851.19
OCBC  337,892.86
RHB  506,576.39
UOB  380,872.02
CIMB  466,830.36
AFFIN  509,809.52
 
 
"2. The best bank to get the highest loan would be HLBB and RHB. However, each bank has it's own ball game
Different bank will calculate your income and debt accordingly based on each bank's different policy. Hence,
I would need to do a due diligence on your profile before suggesting the best bank to proceed with." 
 
"3. I would need to check you CCRIS, CTOS and income documentation before giving you any assurance.

If everything goes fine, 90% shouldn't be a problem for you." 
 
4. If you need my help, do feel free to contact me. I will be all ears in guiding you towards this rough path. 
 
5. I have wrote some articles that help guide first time home buyer to get housing loan, feel free to read it at below link: 
5 Tips for employee to prepare for loan application 
 
Few easy tweaks for self employed and freelancer to ensure loan approved 

Which is why I've indicated earlier to you 600k shouldn't be a prole under hong leong bank. It's being repetitive

1. 85% because its commercial
2. Loan amount is based on 85% of the SPA price.
3. You can refinance it for 85% margin subject to your Commitment/Income.

Will need to see ur documents to see if it's advisable to do so since refinancing cashout is calculated within 10 years.
Full flexi
1. Requires current account
2. Monthly fee
3. Prepayment deposit in current account directly reduces principal loan amount thus reduces interest incurred
4. Don't need to notify bank for prepayments
5. No withdrawal fees for prepayment

Semi flexi
1. Savings account will do
2. No monthly fees
3. You may need to notify bank for prepayment / you can also manually transfer prepayment to the loan account without having to notify the bank. Subject to the bank tnc
4. Some banks may need you to notify them
5. Rm10 to 50 for withdrawal fees subject to the bank tnc

Some islamic loan do not impose lock in period so if you intend to settle the loan fast then Islamic loan, otherwise, either 1 is fine.

Opting a higher cashout would mean you need to pay more monthly thus will affect your future loan, you can however use the extra cash out as a downpayment for the new property so you don't have to borrow so much to balance it back.
Normally if you want to cancel your purchase, why would you want to retain the booking fees ? unless you are buying another similar unit with the agent/developer ?
Yeah you can do so, just bear in mind that you will need to pay for a loan agreement since you are transferring to another bank.