Another thing to remember - "turn" = "player turn", unless otherwise specified. Not game turn. So you're fine to shoot even if you have already over watched, just like you can overwatch if you somehow shot in your opponent's shooting phase/psychic phase.
For example, take any one of the powers that lets you shoot as if you were your opponent's model. Does this prevent them from overwatching you that turn also? Of course not. Interceptor is the opposite exception, allowing an out of phase shooting attack but they still have to specify that this means that you CANNOT shoot in your own shooting phase. If that was the default way the game worked, they wouldn't have to specify this. Or it could just be GW writing redundant rules, but I've never doubted how this works.
And you're correct about the first question. A soulburst prior to the shooting phase (or earlier in the shooting phase) precludes the unit from taking their normal shooting action. Means that you have to be careful about the order in which you shoot things.
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