1. Not by default, the games are played in the web browser, so your options are a little limited. But I supposed you could save the game to a WebStorage variable or save them on the web using the Ajax object.
2. It's a good tool for making games. Nothing in Construct is targeted towards a specific game-mode. It would depend on what you do with the RPG, and how big it is.
3. Dunno, I would say get a credit card, and use PayPal.
4. I don't know of a way to encrypt the data (that would be easy). It would depend on how you store the information. For the most part the graphical and text resources wouldn't be editable (on a webserver) I suppose a user could edit the cached files, but that would just be until they connected to the website again.