Assalam aleykom, regarding the allowed form of lying to ones spouse.
Does it include saying kind words to ones wife like calling her "my heart, my princess, my soul" etc. - Are words like these allowed and come under the ruling of allowed lying if ones says them inteding to achieve a happy home/relationship with ones spouse? Since they obviously are not literal can a muslim still use them in order to achieve a happy relationship/love between eachother? JazakAllahu khayran.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu 'alayhi wa sallam, is His slave and Messenger.

Such statements, which are intended to foster affection between the spouses, are permissible and do not fall under the category of prohibited lying. Rather, they are included in the permissible lying that a Muslim may resort to given the concession for it granted by the Sharee‘ah. Al-Khattaabi said in Ma‘aalim As-Sunan: “As for a man lying to his wife, it means that he makes good promises to her and shows more affection for her than what he actually feels in order to retain her love and rectify her character.” [End of Quote]

An-Nawawi said in Sharh Muslim: “With regard to his lying to his wife and her lying to him, it is intended to mean lying in the context of showing affection and promising what is not binding and so forth.” [End of Quote]

Al-Math-hari said in Al-Mafaateeh fi Sharh Al-Masaabeeh: “It is permissible for a man to lie to his wife regarding the matters that foster love between them, like, for example, saying to her: ‘No one is more beloved to me than you.’ Likewise, it is permissible for a woman to say similar things to her husband.” [End of Quote]

Allah knows best.