Answer 1

nope,

You got daily xp only when you opened to app.

The rest, may be from something else.

Answer 2

yes you have to open sololearn for daily xp

Answer 3

No you won't get any XP. You only get that if you at least open the application.

Answer 4

maybe, someone responded to your challenge request the next day and yiu got some XP for the challenge :)