Adriana keeping her maiden name is a given.maybe mom kept her old name and layla is from current marriage and lexi from prev (my 5 cents) and restaurant reservation was made by current hubby so they are called as monroes.
if Santiago is no longer with her, though, it means Alexis changed her last name to the new husband's. like MC. but then during the beach double date, she would (should?) have spoken up when they were discussing MC changing his current last name, telling her own story to help him make a decision or at least give him an example. there might be a plot hole here.
at least the big picture we saw when MC first met Gabi was definitely Santiago. it's also him in the pictures of her youth. so he is a very important guy in her life.well they aren't but that doesn't mean that nyx won't tease players about them being 1 and same.