If either your power or volume button or mute switch are disfuntional, non-reponsive and/or not functioning correctly, you likely have damage to the ribbon cable that holds all the button components and switches, as well as the headphone jack. Damage is usally due to either impact, Case Damage, or water damage. To evaluate water damage, peer into the headphone jack and see if the water marker is still either white or silver (If red, water damage has occurred).
Preventative Measure include water resistant cases such as the otter box or a body-glove all rubber case for water damage (not fully protective), as well as any case to prevent impact damage that might cause the case to become separated from the back plastic.
If your button(s) or switch(es) are missing, more than likely it is due to case damage, usually from a side impact around the button. Examine the casing around the button and see if there is separation between the chrome bezel and the plastic back of the 3G, if so, you will likely need that repaired or replaced with a new iPhone 3G Back.
Preventative Measures for this type of break is using a case that wraps around the iPhone, or even covers the buttons with rubber, such as an Otter Box™
No. They are the same size, the look the same, but they are not. The 3GS includes extra abilities that the 3G does not have, and thus are different.
Yes. It comes with a 1 Year Warranty against manufacturing defects or installation errors, including the part and labor to replace it. Please ensure that you pick up a warranty card at the time of repair. The warranty explicitly does not include any damage to the back case or cover that is cracked or broken in any way, or if there is any water damage to the iPhone, or if the iPhone was repaired by any other company. Warranty validation may require further inquiry than initial inspection.