Question 1. Does the VF add to the work load of thr TR in the hover?
Yes - there is some blockage of the tail rotor thrust. Look at some conventional tail rotor aircraft such as the A109E or the Kaman Sea Sprite or the UH-60 to see some cut outs where the compromise between forward flight directional stability and hover tail rotor thrust were made.
Question 2. If the answer to Q1 is 'yes' then would a castoring VF be a feasable design feature to unload the TR. By castoring I would have a VF with a suitable bearing in the vertical plane which would allow normal anti-torque operation in the cruise and castor to the airflow (avoiding hitting the TR as it castors) in hovering siyuations.
Castoring would be an interesting idea. But let's think of how much it would castor and when it would have to made 'fixed'. If it castored all the time, it would be ineffective as a vertical stabilizer in forward flight. So maybe it would need to be powered to some position - but which position, and controlled by what?
Neat idea, but it would need a lot of work.
There's a good reason why something like this hasn't been developed or used by now- but I don't know all the reasons.
Question 3. If a castoring VF is not feasable, then an alernative would be a 'Slatted@ VF which should be more efficient in the cruise and then allow air to pass through during hovering.
I think something like this has just been done by NASA, but I don't have any details at the moment.
Aside from the fenestron, the only other attempt to do this was the Ring-Fin from the Bell 400 series, and it appeared to do both jobs quite well. Why it was not taken up on later machines is a mystery.