Your question was closed by 5 users. In order for it to have been closed as a duplicate, the majority of the users must have agreed that it was. That's simply how voting on Stack Exchange sites work - 5 users with at least 3000 reputation on the site (or a single community-elected moderator) can close a question. On the other hand, 5 users (or a single moderator) can reverse that decision and vote to reopen the question if it happened wrongly. That's the way it works on every site on the network, and I highly doubt that will change - it seems to be working quite well.
As for your specific case, let's take it one step at a time:
First, why do you not think it's a duplicate? One of our guidelines for asking questions is to search first. You don't reference any other question on our site about working with legacy code, and there are plenty. How do these existing questions not help you in your situation? You should address in your question that you've done your research and why things that you are finding are either confusing or inadequate for your situation.
Something to be careful of, though, is that if you are too specific, your question might be closed as too localized. If your question is so specific to your unique instance that the answers won't help other people, then the question is too specific. The purpose of this site (and every Stack Exchange site) is to capture knowledge that is useful to a given population (in our case, people developing software, often in a professional environment). Be sure to keep that in mind when reading through existing questions or other resources and when framing your question.
Next, what is the smallest set of information that we need to address your concerns? Your question is rather lengthy, and I think some of your concerns get lost in the details. You mention specific technologies - are these really relevant? You mention specific platforms - are the specific platforms relevant or just the fact there are multiple platforms? There's really a lot to digest here, and a lot of it is extra information. Having extra information means more to wade through and people might miss the differences that separate your question from others if it's buried. Right now, it seems like it's a very common "how do I deal with poorly designed and built legacy systems" question, which is what it is closed as a duplicate of. Why is it not that question, but something different? It seems like there's a very particular aspect of this project you are concerned with, but it gets lost in the noise of everything else.
Finally, there are some comments. The details requested by the comments should be edited into the post. It's OK to respond in a comment as well, either with the information or to let the commenter know your post was updated. However, comments are second-class citizens. Make sure your question reflects all of the information that other people who want to share their knowledge need. It might also help to reformulate the question - the question you mention in the comments is very different than how the post reads.
Given the fact that your post has answers, it probably won't be reopened, even with an edit. We tend to not reopen posts that have been answered if the edits would invalidate existing answers. However, I'd encourage you to search the site and possibly reask. Maybe focus less on the details of the project and the technologies (except where necessary) and on the explicit question you ask in the comments - the feasibility of maintaining two databases.