If I marry my current steward, what happens?
Does the position of steward become vacant after marrying them, allowing for a new steward to be hired?
You get to pay your spouse to clean up around the house... something they should do for free.
Does Lydia offer steward services when out in the field?
Unfortunately, absolutely nothing happens when you marry your steward (as michaelpk reports). They remain your steward, and will go from calling you "love" to calling you "ma'am" (for female Dovahkiins) in one breath.
I tried this with Marcurio. His AI packages didn't reset either, which means that he never moved from Winstad, no matter how many times I asked him to move, where I asked him to move to, or how long I waited. He also continued sleeping upstairs. I haven't found anyone else mentioning this, so either no one noticed this, or, more likely, everyone else had the foresight not to ask their fiance to be their steward.