Hi Henry
I don't have a specific answer for you, sorry, but I few thoughts do come to mind straight away.
a) Size would depend on many things. It is unlikely that a scale keel would be adequate, unless your model is very large, in the same way that a scale rudder needs to be made larger on a working model, because you can scale dimensions, but not the physical effects of wind and water. The smaller the model the more the rudder needs to be enlarged, I would assume the same is true for a keel.
You will also need to add ballast to the boat hull significantly (which would only work for larger models). Because a wherry has a lot of beam, this should be fairly easy, but you could be adding 30lbs of lead on a model of about a meter.
b) Things to consider. On a wherry, they were fixed by bolts and rods, and only removed for shallow water. You may also find you need to use something just to stop the model from slipping sideways. Additionally, no one will see the bottom of the boat when sailing. The other thing to consider is that a lot of scale sailing boats have a non scale keel fin and bulb, because they need them to stay upright. The use of the keel fin and bulb lowers the weight, and increased the righting moment/force significantly allowing you to use much less lead (cheaper and easier to handle the model).
c) On the Thames barges there are lee boards on either side. These are visible when the boat is on the water. Modelers have designed motor mechanisms to raise and lower these as needed. Do searches for Thames barge models and you will find some mechanisms that may be suitable. But again, the wherrys only removed the keel in very shallow water, not a problem for a model really.
Bear in mind these are just my thoughts, I know little about the Norfolk wherry.
