An extension (like a crowsfoot), if used at a 90 degree angle to the torque wrench adds/subtracts nothing.
Using an extension straight out (or at anything other than a 90 degree angle) will need a correction factor. This is because the mechanical advantage is changed by adding length to the end of the wrench (where the torque is registered by the wrench.
A clicker torque wrench doesn't care where you grip it. That only changes the amount of input force required to get the same result at the other end.
A beam type wrench has to be gripped by the handle to register correctly because the shaft deflection is what's registering the torque.