Welcome!
To get proficient I just learned by doing. I have more ideas than I have time to code them, so it is just a matter of trial-and-error. The PDFs are good if you just started or if you need to look up a function. Indicators, screeners and strategies can all be coded as a beginner, but indicators are essential to both screeners and strategies. As a beginner it is easiest to just stick to the default indicators and learn how to take input from them to create a backtest or screener. I have done maybe 2 screeners, 30 indicators, and 100 strategies, so focus on the type of trading style you prefer.
The coding itself is not hard when you understand it. The most important thing is to be able to judge strategy performance etc., and know about potential pitfalls.
Can’t really answer your second question.