In this book you'll learn everything you'll need: https://www.packtpub.com/big-data-and-business-intelligence/learning-qlik-sense-official-guide-second-edition
You could also try taking some courses on Udemy: https://www.udemy.com/topic/qlik-sense/
This one is a must: Book: QlikView for Developers It's for QlikView, but Scripting and Expressions work the same way.
Hope it helps.
Anwering your questions:
1. No, it's not the same. They share many functions, but not all. The Load Script is more SQL-like. For example, you can use the group by clause (instead of the aggr that you'd use in expressions). IFs also work differently. You don't have Set Analysis in the Script.
2. Expressions are the same in QV and QS.
3. Can't understand well your question
There are lot of videos available to learn Qlik Sense on the commuity site and youtube channel.
The Qlik help site is also good place to get started