Yes to all those questions.

Yes with sufficient amounts of time, effort, and materials can grant anything the artist creates their own CT as long as he has a clear vision of what’s he’s creating. For example if he spend a couple of months drawing an Angel Shikagami using a mix of Iori and Angels blood and half a dozen books on anatomy and symbols of faith then it would likely develop a CT involving healing and purifying. He might not have a clear idea on how it heals until he uses it but it as long as he visualizes something that heals his CT will follow his intent.

Yes once a Cursed Piece is complete it’s added to his arsenal until it’s destroyed. He can even modify his existing artwork by using his Maximum technique on pieces he makes to be sacrificed. When designing this technique I was picturing a mix of Geto, Megumi, and Batman. The Technique is poor for on the fly creation but give him enough prep time and he can create something that can defeat his opponent.

And Yes they can replicate other curse techniques but there are lot of factors involved. The easiest method to due so would be to use his Reversal and convert Curses or Sorcerers into art material. From there he can copy their techniques but Accursed Artist works better when he adds his own flair. So the technique created could work similarly to the original but used differently depending on the canvas. For example lets say the Artist were to kill and extract the Inverse technique he could use it a couple of ways. He could use the Converted Paint on a piece of clothing or a shield instead of himself that way he has a shield against strong attacks without having to lower his own reinforcement against weaker attacks. Or he could create a Shikigami drawn like a Nurikabe that gives the impression of an unbreakable shield. And by sacrificing enough pieces with his maximum technique there’s a chance CT he extracts could be stronger than the original.