For now, my solution is to have the HubSpot add-in docked to the right, inside the email window, and have the Grammarly add-in “floating” to the left, in its own window. One can grab the Grammarly section and drag it over so it’s not docked to the border of the email message. It will stay that way whenever an email is opened for editing. It takes a bit getting used to because the floating window will pop up every time an email gets edited and potentially hide another important part of the screen, but at least the text gets spell-checked and sent without errors.
Thomas.