Anytime you make modifications to the a vehicle from how the manufacturer orginally made the car you have the possibility of damaging it.
Yes, putting petrol/gasoline in a diesel engine and running it will cause severe damage to the engine.
Yes
Taking out the two Cadillac converters and putting a straight pipe on your Mercedes e430 will not damage your car.
Turbosmart BOV's are designed to improve the performance of the turbocharger system. They will not usually damage a car engine.
only if it is loud, and it mostly causes slow acceleration
Car battery charges can check the damage done to the actual battery but to the entire car is another story
Depends on the vehicle, If the car has variable timing/lift, it usually limits it's performance/operation to prevent damage.
You can not install multiple performance chip. Most chips use altering of the ecu programing changing the timing and other specs of the motor with two, it would send conflicting information to the ecu possibly causing damage. This is my opinion not 100% fact
Don't fall into those lies. Performance chips don't work. All they do is fool the car into thinking it needs more fuel and forces it to chug gas.
Yes, Mod chips can help. But read the instructions carefully and do exactly as they tell you to. Also consider that if your car is under warranty, using Mod chips normally void the warranty. Check your insurance policy too. Increasing the performance of the vehicle without the companies knowledge might void the insurance as well.
You may do serious damage to the lawnmower if the wrong spark plug is installed.
If by this you mean engine performance chips, then heck no, it will probably break something, if you manage to figure out how to do it, as there is only one port. You can, however, switch, but on a gas car you might need an engine rebuild - diesels are generally exempt from needing modifications.