If he did return, here's how I imagine his and Edith's reunion would play out:
Edith receives a call from work telling her that doctors on their way to work in the morning find a man in rags half dead on the hospital steps, who is identified as Michael. Edith is shocked but is not entirely convinced (for she remembers the burned Canadian officer who claimed to be her cousin), so she goes to London and learns it is indeed Michael, and the doctors mention Michael's body shows signs that he suffered from torture and neglect. Edith brings him back to Downton to get him out of London so he can have some privacy as he recovers. (Side note: they could set the reunion around Easter so as to heighten the theme of rising from the dead)
I like the previously mentioned ideas of him being elated to see Marigold and having a second child after they get married (Edith would then have previously received word that Michael's insane wife suffered a fatal accident at the asylum). And of course his encouraging Edith's career as they co-run and own the company and both publish fiction.