- Is possible to have a player with all the abilities or no matter how much XP you will get you will never have all?

- If I choose an ability that I don't want anymore then is possible to "sell it"? Or do I have to sell all at once? Then I will have all the abilities points restored so I can set it all again?

You're capped at level 20.

When you choose the wrong ability, you only have the "sell all" option, which resets all your abilities, and decreases your level by one. If you're 8, you drop to 7. If you're 20, you drop to 19.