What is the ruling on a husband spitting his saliva into his wife's mouth and her swallowing it with her consent and their enjoyment of that? And is the woman compelled to do so?

A husband swallowing his wife's saliva, or vice versa, is permissible. However, what the questioner mentioned is not permissible except with the wife's consent. Indeed, it is forbidden to proceed with it if she is harmed by it, as it falls under the general prohibition of harming a Muslim. Furthermore, it is not part of the good companionship that Allah Almighty commanded in His saying: "And live with them in kindness."