Sure, I'll explain! But I am on mobile now so a little difficult to explain without drawing any illustrations. If you are not clear feel free ask.

1. Like I said, nowadays most mortgage (please verify with your banks just in case) interest is calculated based on daily rest and not monthly rest. That means each day, interest is calculated based on the OUTSTANDING BALANCE of the day, add added into the interest to be repaid by the end of the month as part of the installment.

2. Keep note that when you pay off the installment, you are paying both the INTEREST and OUTSTANDING BALANCE

3. You want to quickly pay off the OUTSTANDING BALANCE to finish paying your loans.

4. We will follow your example of RM1500 monthly installment: As such, when you pay RM750 on the 15th of the month, and another RM750 on the 30th of the month:

a) From the 16th onwards, you have RM750 less of the OUTSTANDING BALANCE.

b)  The reduced OUTSTANDING BALANCE means that the interest incurred from the 16th to the 30th is now reduced by a small amount.

5. On the 30th, when you pay another RM750, the interest savings you had earlier would translate to a higher portion of the RM750 installment used to pay off the OUTSTANDING BALANCE.

6. The cycle continues and eventually you would end up paying more of your OUTSTANDING BALANCE and reducing the tenure.

Yes, it is a very sound financial planning to do this. This is why some flexi accounts are charged RM5 to 20 a month, because this facility actually has some value to those who use it.

Say, you keep RM20k as your emergency funding all the time for a year in your flexi-savings account, based on 4.5% annual interest rate, you are saving RM900 from paying interests to the bank this way! Of course, your installment stays the same, but this means your tenure is shortened!

Even if you are not keeping a large sum of money in the flexi account all year, but only in the account for a few days between getting your salary and paying off your commitments, due to the daily rest of most, if not all flexi loan accounts, you are saying quite a bit of interests!
Yes, please clarify with the bank just to be sure. It would be best if you could clarify before signing the loan, but different people have different circumstances!  Good to know you are being proactive against paying unnecessary interests to the banks

Ok, good question but if you are compounding the 4.0% FD year on year, you need to compound the 4.5% (RM900) savings year on year too. So since you have saved RM900, please put it back into the loan account and make the emergency fund grow to RM20,900.

Your mileage will vary since there will be cash-flow issues to think about. Why? Because your monthly installment still maintain at a the amount agree and changes only as the BLR/BR changes, as such, the RM900 will come out of your pocket. But doing as I said above (compounding the savings) will shorten your loan tenure even further, and most importantly the savings exist and can be calculated.