I will do a little analogy of how I see it.
Examples:
Saras: Kumud (with curiosity in the car in Dubai).
Saras: Kumud (with affection after reading her poem on Facebook).
Saras: Unhe (to Kussum when she dressed as Kumud to dupe him, he feels the distancing she does).
Saras: Kumud (with love when he realizes during haldi scene how deeply she was hurt).
Saras: Miss Kumud Sundari (Miss Hoity Toity, during fights)
Saras: KUMUD! *Hush!* (Often when she whips up a tornado in his defense in public and she listens! He's more aware of propriety đ)
Kumud: Saraswatichandra (with hope of love, acceptance, before the rejection letter).
Kumud: Mr. SaraswatichandraJI (I have pushed you away, I am distancing you and setting you in your place! Skype scene).
Kumud: Mr. Saraswatichandra (fights, general distancing, balancing their power struggle)
Kumud: Suno (I kinda can't say your name when I don't use Mr. because you are sorta lover, sorta husband, but neither - Most scenes, particularly frustration scenes).
Kumud: Unhe (telling father Saras has to leave after the fire, pretending that a relationship is happening so this is very formal husband term).
Kumud: Usse (with sisters, take ownership by claiming that relationship again, can't say his name because women in traditional cultures avoid husband's name).
Kumud: Tum toh woh ho, Dumbo (I feel like being playful and pouty with you đł)
So they both do a power struggle with the way they use their name against each other. But when they are talking to each other, from Saras' side it comes from love and from Kumud's side it come from acknowledgement of that unspoken/broken relationship. Somewhere in her heart, he has imprinted as hubby/lover and that's why she doesn't call him Saras. At the same time, she doesn't call him aap or suniye because even though he probably has a good 3+ years on her, they have an equal friendship. There's respect, but not formal honorifics.
He on the other hand, says Kumud from the heart like a husband/lover, no honorific, formality, or anything. Just love, be it lover love or hubby love âşď¸