ruthlesslymad

Seat belt main job is to get locked in sudden braking. Check that first by doing semi sudden braking at 40 kmph, then try pulling seat belt (while fastened) VERY FAST and see if it get locked. In either experiment if belt fails then that means seat belt pretensioner is dead and need change. More than likely you need to change the belt as pretensioner isnt available to change. If you find it then its lucky. If its pretensioner is gone and light came up, its pretty justified and it shud be off once repaired after clearing the code. However if belt is fine and still light is on, get the fault code and clear it once and see if it comes back again or no and then decide to repair of change anything.

Option 1: Mechanic must have meant open the engine all attachment on top and sides to reach that hose, while engine is still in the car. Shouldn't cost tops 300 - 400 labor for such job, excluding parts and all. Option 2: Mechanic really mean that opening whole engine is the way then he should ask atleast 2000 labor only. If option 1 no problem, it cud be lot of work involve and he is charging fair. if option 2, i am 90% sure he is bullshitting and better get second opinion that.

Not sure, but it cud be very old and choked AC filter also known as pollen filter. Get that check or changed and see. I am quite doubtful on that. If you smelling carbon smell then you might have exhaust leak near exhaust manifold and CO (carbon monoxide) is coming inside car cabin. Which is very risky for health if you dont fix it it time. Check the first option of AC filter first and then doubt second one.

Check relay and fuses for starter motor, if not the doubt spark plugs, then fuel supply (fuel pump and valve)