Congrats on entering the world of Kalashnimkov Rifle ownership! The Serbs make a great product.
Here are your answers
1: 14x1mm LH
2: The adapters are for all ACE compatable stocks.
3: To the stock or if you got the DF model to the rear stock mechanism.
4: Other folks will have different opinions, but I am not a fan of ACE based stocks. Using screws and loctite to secure the stock in a horizontal position is not as sound as I would like. YMMV.