I googled this for a bit, and while Google did not do a study on opening links in new tabs/windows, it seems like the University of Washington and Microsoft did: http://jeffhuang.com/Final_ParallelBrowsing_HT10.pdf
Just skimmed the abstract, which says "We find that users switch tabs at least 57.4% of the time." but looking at Table 1, it seems like 64% of people do not open search results in new tabs/windows at all, so your initial guess is about correct.
Yes. The vast majority of people do not know how to open a link in different ways. They just click the link. Unless you are making something very techy like github, design for the user that simply clicks on links. Of course, most users expect normal links to open in the current page, so don't just throw target=_blank everywhere.
I'm not so sure they have. Their 'Block all results from domain' feature doesn't register middle clicks. I'd imagine if they had been recording when people open new tabs, they also would show the Block all results link when you middle click.
It's actually quite annoying that middle click doesn't bring up the link.
Just skimmed the abstract, which says "We find that users switch tabs at least 57.4% of the time." but looking at Table 1, it seems like 64% of people do not open search results in new tabs/windows at all, so your initial guess is about correct.