Why would you ask half of these things? A majority of these sentences are wonky constructions that have no real-world relationship or connection. Using your same logic disqualifies the sentence . . . "What is the lion doing? He is eating. . . . When it's patently obvious "he is eating." This also supports my assertion that you're no more likely to use this in conversation than many of the other made-up sentences.
Это is always used when saying "This is something" or "These are something". The various genders and the plural are only used for saying "This something" or "These somethings". "Вы думаете, эти тарелки?" I don't think makes any sense at all.
I'm fairly certain that's right, but if not I'm sure a native speaker will correct me soon enough.
I may simply not understand what the question was getting at in the original Russian, but for some reason "Do you think these are the plates?" was not accepted as an answer. As many unanswered comments in this thread have suggested, the supplied response seems like a very strange one, as it would seem normally obvious whether or not something in view and under discussion were or were not plates, whereas specifying particular plates would make perfect sense in English (are these the ones they sent us to find). Since Russian doesn't use articles, maybe it doesn't seem uncomfortable to a Russian speaker to have one left out in English translation, even when it seems rather needed by a native speaker?