First, the Map Seed question.
When in the new game lobby, "Host New Campaign" from the main menu make sure the first tab "Map" is selected.
The "New Map Seed" button in the middle will generate a new starmap and new bonus ships for you to pick from.
Note that on the simpler maps, it can happen that only the bonus ships and location of the ships change, the map itself does not.
Second, selecting homeworlds.
Left-Clicking a bonus ship will tag that as your homeworld, Right-Clicking will untag it. When a planet is tagged, it's fill color changes from gray to your player color.
This way, you can left-click multiple planets to start with multiple homeworlds. If you left-click no planets (or untag all you have selected with right-click) the game will randomly select a single homeworld for you to start at during game creation.
Hope that helps.