I only answered a few questions below. :-)
> 1. Some people say "why do they not use English".. How different is a sign language from a spoken language
Sign languages are very different from spoken languages just as spoken languages are different from each other on all linguistic levels. Obviously the word level is different because someone who does not know sign languages at all cannot usually understand more than a sign or two if they are lucky. The sentence and discourse level are also very different because sign language make use of space, and therefore can place objects and subjects with more precise relations that prepositions usually can in spoken language. (And this isn't not limited to physical aspects, but abstract as well.) This also doesn't even take into account that sign language have their own way of saying things, including their own idiomatic phrases.
While it is true that many sign language users can usually read English, the same is true for many other language users as well. However, just because one can read and even write in English does not mean that one can do so with proficiency and easy as can be achieved with the native language. The same is true for sign language users. All too often native sign language users will either ask a more native English speaker to explain what a text says or just ignore the information altogether as inaccessible. If the text were in their native sign language, this would not be the case.
> 4. Can you recognise what sign language it is from a written text
I can only because I have had a lot of experiences with various sign languages both written and in person. If a person had no experience, they would not be able to do so. The same is true for spoken languages as well, but seems to be less frequent only because people are exposed quite a bit to written text in other language more so than they ever have before.
> 8. Are there many schools where they teach SignWriting
This is almost an impossible question to ask because there are many teachers who will decide to do it on their own and with various levels of use depending on how much they feel is needed and how much they can get away without the administration telling them to "stop wasting their time" since people don't always see sign languages as their own legitimate languages. Most people would be surprised to know that I use SignWriting in my ASL classes. In fact, I have made it a large part of my classes this year. This isn't because I don't want people to know. It is because everyone involved (except for me) thinks this is how everyone else does it, so they don't tell other people about it.
That is assuming on the individual classroom level. But if you are asking on an entire school level with administration backing, my guess would be very few. The main reason is because most school administrations are not run by users of sign languages, and most view instruction in sign languages as a waste of time. So if they aren't using sign languages, why would they even begin to think about SignWriting!
> 9. How hard would it be to have the pupils at these schools write two articles a month ... How many Wikipedias could be started that way
I can only say for my classes because I honestly don't know of many other that are teaching SignWriting to where students would develop enough skills to even begin writing an article. Because I am teach American Sign Language to beginners, the thought of writing an article in ASL would be too high of an expectation for them, even if it were a class project for one whole semester. If I were teaching more advance students, it might be more plausible, but it would also depend on how much skill they have writing with SignWriting.
> 10. Why is Wikipedia strategically important for getting more people to know about SignWriting
Personally, I think one of the big reasons is that Wikipedia is a website that is very well know by other people, and it isn't associated as being something for SignWriting. Basically, people know what Wikipedia is. If they were to see that sign languages were written within Wikipedia, they would more readily understand what SignWriting truly is, a written form for sign languages. This would apply to both users and nonusers of sign languages.