No.
Yes. You can drive that vehicle with either a Class B or Class A CDL.
Only if it's a: transporting hazardous materials or b: a bus designed to transport more than 15 persons (including the driver).
Not unless it's a bus designed to transport more than 15 passengers (including the driver).
Only if it's transporting 16 or more persons (including the driver) or sufficient quantities of hazardous material to require placarding with HAZMAT warning signs.
Yes
That depends on which country you live in.
If the truck and trailer are not equiped with air brakes. If the truck and trailer are not comercial vehicals.
26000 lbs of cargo? A tandem straight truck, such as a flatbed or a dump, could haul that amount of weight legally.
Air brakes have absolutely NO effect on whether a vehicle requires a CDL or not.
No. Air brakes are not a factor in determining whether or not a CDL is required in any state. The only instance in which you would need a CDL is in cases where the vehicle is transporting an amount of hazardous materials which requires the display of placards, or when it's a bus designed to transport more than 15 persons (including the driver).
This is a tricky one. The GVWR doesn't call for anything but a Class C driver's license--the cutoff for that is 26,001 pounds--but this truck has air brakes and you're driving it as part of your job--maybe your whole job. If I was driving this truck I'd want a Class B CDL and I'd take the air brakes test.