Hello zacaria, welcome to the forum.
You probably won't really like the non-answer that I'm going to give you, but here it is anyway, based on my experiences in the Japanese martial arts ...
You are setting yourself up for frustration by looking for concrete answers to questions on Japanese language and concepts. Japanese language, and thus their outlook on concepts such as mushin, tend to be very fluid and will shift depending upon the context within which they are used.
The answer to all three of your questions is "it depends". For the first two, it depends on how well you've practiced over 8 years, how good your instructor was, what exactly you've been practicing, and who you are comparing yourself to. For the third question, it depends upon the context since it's a Japanese concept. The most common usage I've noticed in the martial arts refers to the ability to respond without thinking. Having responses so ingrained in your body that your body can move and respond properly without having to stop and think about it first. However, it can also refer to the ability to face an opponent without preconceptions, and turning off your conscious thoughts in order to simply react to the opponent. It can also refer to the moment when a person gets confronted with violence. This causes an adrenaline dump that can shut down the thinking part of your brain causing you to react on instinct.
My advice is "don't worry about it." Instead of trying to define things, simply train and let the definitions and concepts take care of themselves.