Re: Are you invincible if you possess Aiki?
There is much discussion and conflict of opinions here that I must admit I have skipped ahead of as it is largely over my head at this point. Simply I will share my beginner opinion on the topic - for what it's worth.
When I first entered my dojo I was full of questions, I literally had a sheet of paper with what must have been 20 questions with which I interrogated my sensei. Many of these I had come up with from browsing here and other online resources regarding Aikido. One of the constant mentions that I had come across was thus, aiki or ki, both as a mindset and as a teaching. My sensei was bemused but humoured me my questions but when I arrived at "how much focus do you place on ki?" he was quite brief. He doesn't know what it is (even after 17 years training) and doesn't teach it. The more I experience Aikido the more I agree with this approach. Just the meaning of ki is very largely conflicted and nobody can really agree on a strict definition, there are biomechanical aspects that some try to teach (i.e. the unbendable arm) but there is also a strong emphasis on the mentality of it.
By now I have trained with a few hundred different aikidoka, from 4 seminars, visitors as well as the large membership of my dojo. I have admired many practitioners but so far I have not come across anything that I would deem mystical in the ilk that ki is held. They can all be defined. Some people train fast, some slow, some people I have felt a great connection with where we synchronized beautifully (flow) and others did not match well. Some people are graceful, some people possess great strength and power. Some people are good, warm, kind, peaceful and "aiki" and others resemble none of those traits. Many people vary on these scales as well depending on the particular day, perhaps if their mood is poor or good, distracted or not. Everyone that I have trained with has had tangible traits, I could see and feel what it was that I "liked" or "didn't like" about their aikido, typically it was a mix of the aforementioned. I believe that regarding aiki, it is as much a question such as "learning patience" from Aikido, sure, it can help, but at the end of the day if you are an impatient person just doing Aikido will not fix that. Striving to improve to fit that ideal I find admirable, but changing oneself on a mental level (particularly if it is extreme) cannot come from the physical alone, no matter how many ikkyo or ki exercises you do.
So in general, I think it can be quite a frustrating topic to discuss aiki, it is at best elusive and hard to prove effective. I mostly consider it as something like faith, if it improves people for the better I am all for it. Very few people claim to "possess" aiki and I have never had the experience to train with someone who does so. It's a little like trying to grab smoke, everyone has a perspective on what they think it is like or how much importance it has, but it is not tangible enough to be properly defined and outlined.