Well... as Meghan was referred to as 'Markle' (without her first name) on another thread more than once and when someone questioned this and asked if it was OK to call Kate/Catherine 'Middleton' that appeared to be a question they didn't want to answer.
And, as I answered you on the other thread, MM is now a CELEBRITY, it's the life she has chosen for herself and therefore now shouldn't be an issue for her to be addressed as such same as Clarkson, Morgan, Winfrey, Clooney, Markle. As with any celebrity as long as she is being talked about I expect she doesn't give two hoots whether it's by her first name or last name.