TS,
U'll only need to remove the air filter if ur screw driver is too long. Otherwise any Philips less than 5" should b fine.

From fool1988's pic, the firewall is on the left side.

As for the not-cool air con, pop d hood n check d air con fan when ur air con is turned on. If its not, get it fixed. Another way to help is by adding in a 2nd fan in front of the radiator although some modifications might be needed to fit it.

Central locking can be done at most accesory shops. Used to visit 1 at Farlim in front of ol Sunshine s'market. Not sure if he has d aftermarket power window set though.