If you're in a college statistics course and you're doing graphs by hand and not generated entirely be statistics software, the skills you're learning are useless anyway.
Yeah that would be bad practice, industry standard is to run all the tests simultaneously and if something comes out statistically significant make up a narrative then try to split it into 4 papers.
Tell that to the reviewers who constantly ask my wife why she didn't do linear regression in her analysis. She rages against linear regression constantly. But some people swear by it, which i think is weird.
My guess is lower. I'd put the correlation at about -.35 to -.45, so that'd correspond to an R² of .1225 to .2025. But eyeballing correlations is hard.
Assuming it's a correction line, I don't think you can tell from the slope of that line alone as the clustering will matter and correlations are finicky. Now, if it was a regression coefficient, that sexy line can be calculated just by looking at it (although we'd want to know if it was significant, lol).