1. It depends on how may apps you're running in the background, what you're doing at the moment of incoming call or txt. For txt lock into memory with provided script. See OP for instructions
2. See this for more more info
Stable vs Development Builds
3. The old themes will for the most part work, but I wouldn't use the old ones. They are not in the updated file format used by the themer. Mtz so will take longer to theme...also some parts will not theme. Fc may occur
4. I prefer using stock launcher because it just goes with the miui experience. Using another launcher is like buying an iPhone only to overly it with an android UI...but yes, other launchers will work
Hope that helped
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You had me at herro