I have thought about a project like this in the past. Before I answer your questions, I'll give you the main issues I saw.

1. How will these indicators be turned on/off? As self-cancelling would be very difficult to achieve (most motorbikes don't self-cancel, and motorbikes have had lighting for a long time), there's a good chance they'd be left on.
2. They would have to be very bright, and anyway, in my experience motorists are not paying any attention to what cyclists are trying to indicate anyway. Even in bright daylight, with a reflective jacket on, I can stick my arm out to indicate and start moving over, and someone will still try to overtake. I wouldn't be confident that I could safely move after putting on my little bicycle indicator.
3. What kind of bars will they fit? (drop / non-drop). Either way, they will have to have some protruding elements for full visibility, which puts them right where they will get knocked, as well as potentially looking goofy.

Your questions:
1. It can be hard, depending on the junction. Yes, in the rain, I'm more focused on braking than indicating.
2. Maybe, if they can have rear and front-facing lighting.
3. Not really. I'd want them maximum brightness all the time
4. I wouldn't buy them at all
5. I don't know
6. No
7. Yes