Structure a talk around one claim, supported by two or three points, each carried by a concrete example. State the claim early rather than building to it. Avoid chronology as your spine unless the sequence itself is the point. Signal every transition out loud, because an audience cannot see your headings.
It needs a clear opening and a clear close, but the middle should be organised by idea rather than as a narrative arc. Talks that follow story structure often bury the argument in the second act, where an audience is least able to hold it.
About three, from a single listening with no notes. Planning for more does not increase what lands, it reduces how well each one is made. If you have five things to say, you probably have two talks.
No — deliberate repetition of the central claim is one of the few tools you have to compensate for the fact that nobody can re-listen. Repeating supporting detail is different, and usually just means the structure is unclear.