Some unofficial answers here, but these are exactly how I worked on my own assignment.

Q1

According to the clarification email from the professor:

All rows with a country code, between the years 2011 and 2021 are to be reported.

So an empty value like population does not affect whether a line should be outputted or not.

Q2

Yes.

Q3

Yes.

Q4

Again, according to the clarification email from the professor:

That implies that, for a given country and predictor, each Cantril data point corresponds to a a predictor data point, and there are at least 3 of those. This also implies that the number of correlations may be slightly different for each predictor.

So Life-Cantril correlation does not need to be calculated in this case.

But you still need to calculate correlation for GDP-Cantril.

Q5

Yes. For each predictor, n can be different for each entity, for the reason we talked about in Q4.