A straight extension won't affect torque. An angle drive adapter will, though. You'd need a constant velocity joint to ensure the torque on both ends was the same.
A swivel adapter will transmit different amounts of torque and rotation depending on where the two pins are relative to the angle. You can see this for yourself by sticking two extensions into the adapter, bending it about 70 degrees, and slowly rotating the two extensions. With a small angle the error is not too large but as the angle increases the errors increase.
You can also measure torque with a conventional wrench and a spring scale. You have to determine the distance from the center of the fastener to the attachment point of the scale and be sure that the scale is pulling at a right angle to the wrench.
Simple math tells you that (1 foot/distance along wrench)x force on wrench will give you the torque delivered to the fastener.
Using a crowfoot adapter with a torque wrench will make your torque reading lower than the actual torque if its opening is opposite the wrench handle.
The torque sticks used on impact guns rely on the fact that the impact gun rotates a certain number of degrees per impact. Twisting the torque stick that number of degrees will result in the desired torque at the socket end of the stick. Used in a steady state situation (as with a manual torque wrench) they will have no effect on the torque delivered for a given reading on the torque wrench.