What is the difference between preparing a research proposal and writing the actual thesis in the case of theoretical research?
In general a research proposal (separate from the literature review) is a description of the questions that you wish to answer in the research and a description, perhaps preliminary, about how you intend to answer those questions. it doesn't include the results, which, we assume, aren't available yet.
The literature review asked for will probably be less complete than the one that later appears in the dissertation, since all that is needed here is the papers, etc, that you have read that lead you to ask the question and support the idea that they are important questions that haven't yet been answered. You will possibly need to do more in a formal literature review once the research begins.
You seem to imply in the question that all research questions have already been answered before you make the proposal. That is normally not the case. In the example you state, your proposal should also include how you intend to support the conclusions you reach. This might include additional library research or interviews with knowledgeable people, etc.