Keep in mind that four of the GOP candidates were sitting U.S. Senators. Add Jeff Sessions to the mix as a member of Trump's inner circle, and you've got a fifth one under FISA surveillance -- conducted with foreign agents -- as well.
Absolutely correct. They used foreign Intel services to run the eavesdropping projects.
Probably as another layer of complexity to avoid detection, plus domestic surveillance is easier to detect, sercumvent, and track.