Using a DT bookmarklet in Safari causes the focus to switch to DT. I understand that can’t be fixed. However, if you’re going to be using the bookmarklets a lot then first go to DT, display the global inbox, then minimize that to the dock. Now, in Safari the page you are working on will stay frontmost on the monitor, even if the focus is actually on that minimized page in the dock. The cursor remains where it was, so just click and you’re back to Safari.
Or just press Cmd-H after using a bookmarklet.