I want to search for all messages in my inbox from a particular sender and archive all of them (or possibly the ones I select). It appears that archiving isn’t an option on search results. Both the button and the Action menu item don’t show up.
button “Archive” on toolbar is shown only when server supports archiving. Anyway I understand that you are trying to archive from “all inboxes” folder, that is not possible because this is just a virtual folder.