Hi bro, thank you
1) so it ok to switch if "OFF"(SEEM IT NOW SERVE NO PURPOSE)
2)You can test to find out from your main switch by switching off individual 20A or 32A MCB switch.
This test for what Purpose?
3) which better connect to isolator or 15 amp switch only( as might as my installer to rewired if it make a difference in long run)
thank again for the input ,appreciated
the isolator is supposed to serve a purpose, i.e. to isolate the power to the equipment when doing maintenance. when technicians do servicing of the ac, it is safer to isolate the power to the entire ac system as there will be spraying of water, opening of cover, etc. although you are probably not going to service the ac yourself, i'm sure it is in your interest that no mishap happens during servicing of ac.
cis2910 has answered your queries on why you need to know which breaker is controlling the power to the ac. so based on the para above, your breaker is sort of performing the role of the isolator. but in practice, it is located far from the cu itself. and it is actually not appropriate to use a breaker like an on/off switch.
you can isolate a 15amp supply with a 20amp isolator, but if your mcb is rated 20amps, you should be using at least a 20amp isolator as you might fry the conductor at the 15amp plug/socket.
back to your query on why your ac is still working when the isolator is in "off" position. it could be, as other forumer mentioned, that your ac is not wired via that isolator (for BTO, there's another 20amp heater point termination at the bathroom that the installer could have connected to). or in an extremely extremely extremely rare situation, the 'live' conductor is wired via the earth link, so switching off has no effect, and the installer should be shot if he does that.