In short, your question is perhaps too long. While I do not personally think it is, I find that many users 'get bored' after reading the first paragraph. This can often lead to assumptions or responses to only the first part of the question. Your question is a good example, where users had posted, without clearly reading the second half of your question. That is likely why users suggested options you had already covered.
I find that if I am writing something long, I try to summarise each paragraph in its first sentence. If it can stand by itself, I sometimes put it in bold. Keep in mind that lazy readers are more likely to only read the bold.
Using markdown can help break up your body, too. When you mention options you have already tried, for instance, you could try using a list.
- I have no prisoners to ransom
- I have already tried asking the pope for money
Holding back on commentary helps a great deal. I have no experience with the game, and can not tell how much of your body is relevant to the question, directly. But things like "For some reason I can't understand, I got to keep only 1 county out of 4, the other 3 went all to the second son. Gavelkind is strange. But this is not what I'm asking." could easily be summarised as "I only kept 1 out of the 4 counties". It is a lot less to read, and it pushes the point across.
If you think you are at risk of having a long question, consider how to summarise it in one or two sentences. If I can, I put a "TL;DR:" at the top of my body, that summarises the point. This can do more harm, as appears the case with you, as some users will only read the "TLDR", and post an answer that conflicts with secondary information.
Some questions are good, but just don't hit the right audience. A good way you could recover your question would be by posting a bounty on it. A bounty can attract high-quality responses, and you have a lot better chance of people reading the entire question. If nobody has posted a helpful answer by the end of the bounty, you can simply not award it, or even up the reputation reward and start over.