Hi,
1. That looks like either another plugin stylesheet or a theme stylesheet is conflicting. They should not be loaded on 3rd party plugin pages, it is usually a very bad practice from developers. Unfortunately we cannot prevent that. I recommend trying to turn off other plugins one by one to see which is causing the issue. If none, then swtiching the theme temporarilty.
2. You can check this documentation to optimize the settings. The rest is up to the server performance.
3. Check the grouping documentation for possibilities.
4. The screenshot you sent is the same as on 3., can you please check? Thank you!
5. You can change that under the General Options -> Logic & Behavior panel.
6. By changing the post type results limit under the General Options -> Limtis panel, and also enabling the more results feature.