I am guessing that you took the gas cap off during your petcock testing so we can probably eliminate a pugged gas cap vent. If you did not, try it.
You will have to take the petcock off to check it out so you need to catch the gas in something when you do. After you get the petcock off you can check it by just blowing through it in different positions. Depending on the type of seal it uses internally, the seal may have slipped in which case, you have a dud and the flow will be affected.
I am also assuming you do not have mass quantities of crud in the tank.
The choke/no choke problem is because you do not have enough gas to run.
On the throttle problem, there are only three pieces. The twistgrip, the cable and the carb.
You can check the carb by lifting up on the cable where it enters the carb, similarly, you can check the cable by pulling on it where it exits the twistgrip.
The twistgrip should be greased, the cable should be oiled with motor oil and the carb should just be clean. You can squirt WD40 in pinch but it is not the best lubricant. You may also have broken strands in the throttle cable which means you need a new one.