The Core Sound 20 can certainly be rowed, although other boats are no doubt more satisfying if rowing is the primary means of propulsion. I have made several multi-mile rows in the 10 years of sailing without a motor, but this was always because the wind had given out completely. Many times the wind would die to dead flat, I would row a few hundred yards and then the wind would fill in around the headland. The sails fill easily on a light breeze, and I found the boat needs very little wind to sail faster than I can row it.
A sliding seat would no doubt be nice just forward of the mizzen, but only if the intent is to seriously row this boat just because you can. For my style, with sailing being the prefered mode, the sliding seat would be unnecessary clutter. I have rowing stations both for and aft of the mizzen. I use the aft station most, typically rowing from a standing position while facing forward with the tiller between my knees, and the sails hoisted. As soon as the breeze filled the sails I quickly ship the oars and settle in for the satisfying ghosting of this boat taking off on a faint wisper of wind.
The boat is better balanced for long distance rowing from the station just forward of the mizzen. With the sliding seat pay close attention to how far aft your nose ends up, as the boat will want your weight close to the mizzen. I imagine with the Mk2 the need to push the rowing station aft will be even stronger. Consider running a steering line around the cockpit so that you can steer with the rudder from to forward rowing position, helping to quickly transition from row to sail as mentioned above. For long distance dedicated rowing you will want both the rudder and centerboard up, and the rudder locked midship if not removed from the water - provide a way to to that. My boat tracks nicely while rowing so the rudder is more drag than tracking benefit.
I found 10 foot oars about right for me, but trimmed to 9'5" to allow the oars to slip under the thwart for storage in tubes running up thorugh the forward bulkhead. If you go forego the aft lazarette locker longer oars will likely fit.