Combo questions... what fun!
Question 1: Is there a standard for loading data? Load on demand per section or all together?
This is a "it depends" type of question. The rule of thumb I'd use is to load enough data to be useful for the user but no more. More data can be useful, but it does mean a longer loading time.
So, the question to ask:
As a user, what's my typical interaction within the app? Am I more likely to dive into a specific area in the app and move on? Or am I more likely to bounce between multiple areas because the data between sections are highly related?
If data within each section is fairly discrete and users are likely to only interact with specific sections, then scenario one: loading data on-demand is a better solution because the load time will be reduced.
If data within sections are highly related, and users are expected to jump back and forth regularly, then you may want to consider scenario two with pre-loading all associated data. This make sense because there's a cost in section switching. The longer the wait when moving between 2 related sections, the more likely the user is to forget the connection between data in one section and another.
Question 2: How to deal with refreshing real-time data?
So you have an app that displays real-time data. When should you refresh the data?
Question you should ask: As a user, how important is it to get the most recent data as compared to waiting for the refresh to happen before you can view the screen?
If this is like a stock market app, where only the most up-to-date data has any value, then you probably want the system to auto refresh this for you when you return back to the app. If this is something like a RSS feed. Then having the most recent entries isn't your friend. Not only is the wait annoying, you probably want to return back to the last article you've read and not have the system load in 10 new items and make you lose your place.
What about if the user is currently looking at the data and there's updates?
The user is viewing the data, so you don't want to auto update it on them. What you should do instead is to display a small message indicating data has been updated. This can be dropped from the top just beneath the pull to refresh. This way the user can choose to update should/when they want to.