You seem to be describing a problem that needs to be solved rather than a situation meaning that there is anything wrong with the technology. I used to have speech control turned on on my computer - you can set it to listen to everything which is basically the situation you describe with all its problems. Alternatively you can ask for it to listen for a keyword or only to listen when you press a key. I imagine similar solutions will present themselves for brain-computer interfaces.
I think there is some truth to that but I'd say the one difference between speech recognition and brain recognition is that speech is a voluntary action you control, while your thoughts have a largely involuntary component to it. Involuntary meaning when someone says "an idea just popped into my head" - the idea seemingly was not an action deliberately triggered. Think if while you were "mind typing" an email and suddenly the thought "god I hate my boss" popped into your head. If the filtering mechanism of the computer was poor, your computer might, assuming it was being helpful, shoot off an email to your boss saying "god I hate you". I guess what I'm saying is the way by which you could filter your own thoughts to be interpreted by your computer or not seems like an incredibly difficult proposition.
Yes, but I think we can also distinguish, in our own minds, the difference between a thought that is fleeting or passing and a thought that we want to take action on. Similarly, a successful brain-computer interface should be able to make that distinction.