maybe i can help to englighten on this;

there's a different between a semi-flexi and a full-flexi.
1) Full flexi = housing loan acc linked with current acc
2) Semi flexi = housing loan only (standing instruction or schedule payment from current acc is not consider linked, but more of an instruction to transfer fund from c/a towards your loan account on a specific schedule.)

explanation;
scenario 1= a full flexi mortgage structure is linked with a current acc, in which the balance in c/a will be taken into consideration when it comes to interest calculation on your principal. I'm suprised that the officer says "no" cause theoritically, even if there is no extra payment made towards the loan acc, the balance in your c/a will still be taken into consideration as principal reduction.

by right, it doesnt matter if the additional is made towards the loan or the c/a, it is still principal reduction if its in either one of it. by left... i wouldnt know how it works there...  (check with her again perhaps?)
scenario 2 = semi-flexi has only 1 account. in order for your to reduce your principal, the money MUST credit into the housing loan account (subject to LO TnC). any payment made to your c/a has nothing to do with your principal as these accs are not linked. so the 2nd officer is "korek". =)

hope the above helps.

Not sure I understand what you mean. Do you mean, when you have additional, let's say RM10K, you park in the bank.

1)But the bank don't automatically deduct from the principal thus reducing interest?
2) Bank only consider the money as monthly installment to deduct every month.

For eg. loan 100k, park 10k, interest should be 90k only, but UOB bank does not charge at 90k, but charge at 100k instead. That what you meant?

whether its semi or full flexi, its more or less the same thing. just think of it as a manual car or auto car... higher FC and up-keep fees for the convenience vs more leg work with lesser maintaince cost... relatively speaking. 

from personal point of view, i'll say go full-flexi if you have the additional cash and IF you going to roll the money very often.

else, stick with semi-flexi as its more straight forward when it comes down to numbers.