Yes, they are different forms of the same word, 'ons' or 'our'. You use 'ons' in front of 'het' words and 'onze' in front of 'de' words and plurals (from both 'de' and 'het' words)
I think it would depend on whether DL accepts UK English. In US English it's breakfast, (brunch), lunch, then dinner. So, dinner is the third main meal at the end of the day.
I added brunch in parentheses as it's either a late breakfast or early lunch and replaces either of those meals.