Yes chaudhary jee... idea is good but I was also thinking one reason of not placing speaker under dash could be because alarm sound would be as loud as it should be due to closed cabin... but solution to this could be installation of speaker on some other place like under the engine block, under carburetor or some other unusual location or it could be enclosed within a steel box connected with firewall securing its wires also... may be you can think of some better place and share with us...
Yes 2 way system is far more better than normal alarms as you will not know whats going on with your car.
I think this is possible unless its programmed into ECU. (carby cars don't have ECU so these might not benifit)
Manual should be the best place to search for if alarm system has pressure sensor or not... normally its a microphone which senses cabin pressure and detects variation in pressure when cabin door is opened or closed.
Tilt switches are to provide safety from thief towing your car with his tow truck. This is basic operation of tilt switch..
Mercury switch - Wikipedia, the free encyclopedia