Keeping the spoiler space, but really at this point, if people have not seen the movie, they desrve to have it spolied when clicking on a thread about the movie.
But anyway, read nothing into the scene with Violet and Mary. Maggie has been quite vocal in the past about being happy that Downton was over, saying on one talk show “By the time we finished, she must have been about 110”.:
https://youtu.be/rvrMVf8HJ44?t=173
And yet she’s back. Additionally, while the scene gives a potential “out” for her if there is a next film, Maggie herself changed the dialogue from the script from “do not” to “may not.”