Yes, you can add as many sliders as you need to one page, with different skins, options etc.
No, you can get all updates of RoyalSlider completely for free.
RoyalSlider is built in best practises of WordPress plugins and works with absolutely any theme.
Absolutely not, RoyalSlider caches all sliders using WordPress Transients API.
Make sure that you unzipped archive that you downloaded from CodeCanyon, inside you'll find readme file.
Contact me with detailed explanation of your project and where you want to use slider. If I agree with the way how the product will be redistributed and how it looks like - I will provide written permission.
For each website new regular license should be bought. Read more about regular license on CodeCanyon.
No, it's still for single end product, please refer to Envato licensing FAQ.