"Я не знаю, что мама приготовит завтра на обед."

Certainly.

I think it is partially a situational difference. I made the same mistake while reading this too, but I believe in order to make the sentence into "i don't know , that mom will make lunch for tomorrow" it would probably have to be "i don't know , if mom will make lunch for tomorrow". "If" would fit the sentence better than "that". example “I wonder THAT this is wrong” & "I wonder IF this is wrong”. Don't worry many English speakers make this mistake, even me lol. good luck :)

What you say may be correct for about 15% of Americans (I figure I expect is ever decreasing), but not for the majority: https://www4.uwm.edu/FLL/linguistics/dialect/staticmaps/q_96.html

Historically, there may be the idea that "dinner" is the main meal of the day whenever it happens to be eaten. I think this idea has been broadly lost in American English at least. If one has one's main meal in the middle of the day, I think by far the most likely statement would be that one eats a "big lunch."

For my nonagenarian grandmother, "dinner" is the noon meal. It is also the largest one in her retirement home (part of the reason she picked it; she wants her main meal in the middle of the day). However, the retirement home calls this meal "lunch" on its printed menus.

Not necessarily. There's a lot of variety in how these words are used in English. For a lot of people, обед = lunch, ужин = dinner.