Hi,
1. Because you have configured to only highlight whole keyword matches, you can change that here: https://i.imgur.com/lgXFDgn.png
2. That is not possible unfortunately without massive custom coding.
3. Because of the grouping and of the large number of results limit, some seemingly more relevant matches can possibly be lower in a different group. You can turn on this option reorder groups by more relevant results first.
However that may not change much, because of the high number of matches it is still possible, that a group has a higher average relevance.
To get results from the most relevant to the least relevant is best to have grouping disabled. Because if the results are already grouped with large number of results, then it may lead to unneccessary long list of results.
A good alternative is to use the advanced title and content fields – which allows you printing the category/taxonomy term names before/after the description or title fields. Although it does not group, but it is a nice alternative to display the result categories.