Logic gets thrown out the window in the face of the author needing a plot device in order to develop a romance.
I mean technically she needs a dude to get her pregnant so she can be a nanny to the new heir so successful plot device ? Princess wants her to bea their nanny but she also wants her to have a good marriage so it works.
I think the King and Queen wish for her to a wet nurse/nanny for their future child(ren). You kinda need to have just given birth to fulfill that position and it would look poorly for all parties if she got pregnant out of wedlock. The King and head guard were milk brothers (ie nursed by the same person). Mary and ML seems somewhat close in age so probably appointed during childhood, same as ML with the Queen who was 9(?). Obviously at that age you don’t need a wet nurse.
I think it would be interesting and sensible for her to live without marriage. Her own nanny, Mary, isn't even married and still raised her. Why can't she do the same? Also, where the hell is the prince? He needs to collect his wife.