In order to repair these do I need to use primer and paint?
should I change to a better compound for the middle crack?


It isn't about a "better compound", it's about the right product that works well for you. Your product is closer to spackle (a hole patching product) than drywall joint/topping compound. Although stronger, it is not surprising that it is more difficult to sand. Your product will work, but you may spend more energy and time getting it smooth and flat. If you hate it, try using regular joint compound. Joint compound is not as strong, takes longer to dry, and shrinks when it dries (requiring repeat applications).; however it is very easy to sand, and goes on creamy and smooth from your knife.


  Do I need to use primer and paint after compound


Yes, unless you will accept obvious patch marks on your wall and understand that the repair will be easily damaged if touched.

You need to apply several layers, each successively wider than the previous, and sand flat between each layer, if you want the patch to look good. Even then, it takes experience to create a totally unnoticable repair.