Welcome! Below are my thoughts on your questions - hope it helps:
Rudder - you can get various shapes and sizes but your 8" sharkfin is the most popular as it is a good mix of directional stability and hydrodynamics. The position of the pedal is a personal preference. I prefer to have my pedals just back from vertical i.e. leaning towards me by about 1cm from vertical so that I still touch the pedal with my outside foot when turning without having to overextend my foot. The overextension can also lead to cramps if the pedals are really far back on both sides.
Scuppers - the venturi on the boats that I have had in the past usually only started working at 8km and above (my V12 has the dinghy style which works at about 6km). Not sure what you are talking about re the vent tube? The scupper should have an opening in the cockpit and that opening should run unobstructed through the hull and out the opening under the hull. When your ski is on dry land fill up the cockpit with water and check that the water drains with no obstruction. If that works then you can consider some of the improved shapes that some manufacturers have on the hull to creaqte a more powerful venturi.
Wing - throw away your Lendal (or sell on eBay) and get a wing. The wing will make your life a lot easier as the shape of the wing guides the paddle through the air and the water making the stroke a lot easier and more stable.
Lastly - get a copy of Dawid Mocke's book - it will really speed up the learning process and improve your paddling skills.
www.surfski.info/reviews/item/1358-surfs...i-with-the-pros.html