You really can come up with a lot of them. Hehe

1. Where were you going to write this? There is a question at the end of the questionnaire form ("On a seperate sheet of paper, provide any additional details ... ), I'm assuming you meant it for that. And I'm going to guess that is the summary of the short version.
I wrote a 2.5 pages long letter for this and in it was the summary of my 5 year relationship with my husband. This is a great place to explain not only the facts, but also the circumstances. So for example, I would write something like "We met on a journal website in 2003. We occasionally commented on eachother's journal and became friends. We kept in touch, talking using instant messages. We weren't looking for a relationship, but during the summer of 2007 we realised we had developed feelings for each other and decided to attempt a relationship. We continued to talk over instant messages, whenever time would permit it. At the time, we were both college students and luckily our class schedules allowed us plenty of free time and we were able to spend many hours talking each day. Unfortunately, at the time we had not thought too far ahead and hadn't kept any records of our conversations, we never considered they would be needed for immigration purposes. .... "
Obviously I just made that up, but I hope you see where I'm getting at. Make them see the whole picture. The forms are enough to provide post of the facts, this is where you get to mention the things that you can't put on the forms and show them you're a human being, not just some facts on paper.
2. None of the forms need to be notarized. You might need to notarize any court papers you were submitting, such as custody papers or the like, but I don't think you have anything like that.
3. See my answer to #1. You don't have to write a lot about how you love each other and any mushy stuff, but do try to work some feelings into that letter.
You would attach this to the end of the questionnaire form, a paperclip will do. On every paper you attach to any form, write the form number at the top of the page, so they know where it belongs if it gets separated. And to be safe, also write your name and date of birth on it, so it doesn't get lost.
4. I don't think it's going to be an issue. Write a note with the supporting documents, explaining the differences between names and I'm sure it'll be enough.
5. You don't have to submit letters of support. They're not mandatory. We had none, we felt the rest of our evidence was strong enough. If you decide to include them: there's no set form or format. Just have the person write a letter, stating what they know about your relationship and how they're certain you're commited to each other or whatever. It's really up to the person writing it.
6. If you're marrying in October, you still have some time to generate relationship proof. I'd recommend you switch to another messenger, something that keeps a history. Whatever you choose, make sure the logging is turned on so you can submit the logs. Have you tried Skype? It's been working great for us.
Keep in mind, you dont' have to prove every single part of your relationship. So if you can't really prove you were living together, that's ok. Just submit what you have, even if it isn't solid proof. The most important part is proving you have a genuine, continuing relationship, not every detail from its past. By the time you're ready to apply, you'll also have wedding pictures, cards from family saying congratulations, ...
Also, don't forget, you can also submit evidence from other people. For example, my mother-in-law has been sending me christmas cards for a few years. I included copies of those. If you have any emails or letters to or from other people, in which you mention your relationship, you can include those as well.
I have no doubt that with as long as you've been together, you will be able to provide enough evidence about your relationship and your application will not have any issues.