Yes (assuming they're doing frontend dev and including the resources from the page). The code is fetched and executed from the browser, so It'll have to escape the browser sandbox to do something nefarious.
Can be mitigated, as the sibling comment points out, but even in the situation you described, the blast radius is reduced, especially for frontend libs.
Same. I came back to do a little frontend work a couple of years ago and was horrified by the replacement of script tags with subresource integrity with npm and bundlers.