You're talking about an air brake restriction on a CDL, yes?
If the vehicle does no require a CDL, yes. If it requires a CDL, no.
So, you could drive an RV in a non-commercial capacity, you could drive a commercial truck up to 26,000 lbs. GVWR (so long as it wasn't either a bus designed to transport more than 15 persons - including the driver - or carrying a quantity of hazmat which requires the display of placards under CFR 49), you could operate a military vehicle (as a military service member in the course of their duties), you could operate an emergency vehicle (as a member of that force, if the department doesn't require a CDL), you could operate under the farm truck exemption (even if the vehicle exceeds 26,000 GVWR)... but if it doesn't fall under the emergency, farm, RV, or military exemptions from requiring a CDL (and within the limitations of those exemptions), then a CDL is required. Single axles can be a little ambiguous... for example, various model years of the Ford F750 can have a manufacturer's GVWR of 38,000 lbs. (which is what the axles and suspension would be rated for, but wouldn't be road legal under bridge law), but it can be still be plated as a 26,000 GVWR truck, thus alleviating the requirement for a CDL.
is it illegal to drive with an infant in the front seat of a truck
The length from the middle of the steer wheels to the center point between the drive tandems (for a tandem truck) or center of the single drive axle (on a single axle truck).
The transmission
No.
Yes you can remove it and still drive the truck......
Find someone who'll employ you once you have a CDL, and will offer their truck for the road test. It doesn't have to be a dump truck. You could rent a 33k single axle truck from Penske with an automatic and air brakes, and, according to the DOT, it would qualify you to drive a Class 8 dump truck.
Someone who knows how to drive a truck and is legally licensed to drive a truck has to train you, so that you can drive a truck (but you do also have to get a license).
Truck classification isn't concurrent with the license required to drive them. A single axle water truck could be a Class 5, 6, 7, or 8 truck, depending on the vehicle GVWR. A Class 7 or Class 8 truck would require a (minimum) Class B CDL.
The power unit has a steer axle, and only one drive axle.
You could do it but it would be much cheaper to just buy a good used 1990 GMC 4+4 truck and drive it.
It depends on the truck, Could be 4 hours or 10 hours.It depends on the truck, Could be 4 hours or 10 hours.
Not enough information to answer. What type, is it a straight truck or tractor trailer, how many axles, what class vehicle, etc. would be needed to give you an answer. With weight laws in the US, a single drive axle truck rated at 26,000 lbs. gross vehicle weight could carry up to five tons, while a Class 7 single drive axle truck rated at 33,000 - 35,000 lbs. could carry up to eight tons. A tandem axle straight truck would be in the vicinity of 12 to 13 tons, and a standard, five axle, semi tractor and trailer can range from 23 to 25 tons of legal payload.