No, you have everything needed in your chain.
It depends on whether the windows Audio driver (WASAPI) and audio output (depends on the manufacturer) are good eneough for what you are doing. If not the Focusrite might provide better Audio fidelity and drivers for live use. It depends on the system and your sound quality needs.
There are many pianos for Kontakt. So it depends on if you have the Kontakt with Factory Library or one the Komplete series that have higher quality sample sets. These sets are probably better than the sets you have on the iPad.
For piano parts it’s far too small. At least 49 but 61 is better for a small portable.
The problems depend on the user so it’s a broad question to answer here but you can do a search of the community forum by clicking on the magnifier glass on the main toolbar for the site and you will see a variety of troubles and problems along with fixes and strategies as well. When first using Cantabile you can avoid trouble by viewing the Cantabile ‘Getting Started’ videos here :