The above answers sound good to me. One thing I will add is that it's important to make sure in flat areas, that your highpoly is truly flat. So that means enough loops and divisions that the surface does not get curved, otherwise your low poly bake will not be flat in turn.
You'll get different answers to this. I say, if it looks good smoothed, then you're good to go. Generally things like 5 pointed stars, weird ngons/tris etc on a flatter surface won't show up. It's when these things occur on angles that you get issues. On your low poly, however, you'll want any edges to provide silhouette.…
I've always been under the impression that the topology of the high poly is pretty irrelevant as long as it looks like the model you are making, and the edges are smooth enough to transfer well onto the low poly. The low poly is the model that is going to be used inside the game engine, so that is where the topology is…
@2nd image' chamfer that vertex. youl get a equaly spaced square, then use a bridge between corners and in the middle reform it into a square instead of a [x]