"Whose lunch is this?" -> the subject is "this", "this" can be interchanged with "it" ("Whose lunch is it?"), "this" is a pronoun that plays a noun role. An answer to this question: "This is my lunch".
"Whose is this lunch?" -> the subject is "this lunch", "this" cannot be replaced by "it" ("Whose is it lunch?"), "this" is an attribute of "lunch", a pronoun that plays an adjective role. An answer to this question: "This lunch is mine".
It just happens so that in English both roles are played by "this", and in Ukrainian the noun role is "це", and the adjective roles are "цей/ця/це". It's the same in German, "das" plays the noun role of "that" ("Das ist ...") and "der/die/das" the adjective role of "that".
So... At least we're sure breakfast is breakfast :') But then:
Lunch so far is just the midday meal, so обід.
Dinner used to be what we call lunch before, or simply the second meal of the day (so обід), but then it slowly got pushed to the evening it became the evening meal (so вечеря)
Supper is the old word for dinner (so вечеря).
I think it's like this.
My understanding growing up in the US is that "dinner" is the principle meal of the day. So for some people "dinner" is the second meal of the day, or lunch. For others, who eat the larger meal in the evening, that is dinner, or supper. Like you note, lunch is always midday, and supper is always the evening meal. But dinner is whichever of the two is more substantial. I hope this helps.