There's evidence than Strong Bad and Strong Sad got along really, really well. So why is Strong Bad so abusive? Listening to the Marzipan Answering Machine 17.2 and seeing one of the Decemberween videos, you realize Strong Sad enjoys being sad and put upon, which means Strong Bad is abusive because that's what Strong Sad wants.
Why have Strong Bad and Homestar become closer and friendlier? Homestar is the only cast member who enthusiastically indulges Strong Bad in his Dangeresque movies and actually does a good job acting and treating the material seriously.
Why did Bubs resort to violence to get Strong Bad to take back his story of "Orginal Bubs"? Because, as Strong Bad himself later confirmed, Strong Bad is a reality warper whose stories and ideas become real at random, and Bubs didn't want to be replaced by Strong Bad replaced actor parody. And it's a good thing he did, parts of that story that weren't shot down became real, from there being several Kings of Town to Onion Bubs.
Strong Sad writes a poem in which he gives Strong Bad human hands in place of boxing gloves, much to Strong Bad's horror. This makes sense since fingers are appendages Strong Bad is not used to having. Strong Bad is used to typing with his boxing glove hands (by simply bumping his hands on the keys and somehow getting the letters he wants). In the sbemail "fingers", Strong Bad attempts to type with makeshift fingers, with atrocious results, so having to learn to type with actual fingers would very likely drive Strong Bad crazy.
How did Pom Pom come back after getting killed? He previously got a 1-Up by beating the crap out of Strong Bad in the Strong Bad Email "pom pom".
In sbemail206, we get a trailer for Dangeresque 4 / 2 = 6, in which there are two Dangeresques and two Dangeresque Toos. Two Dangeresques (1 + 1) + two Dangeresque Toos (2 + 2) = 1 + 1 + 2 + 2. That equals 2 + 4. That equals six.
In sbemail84, Strong Bad says that Southern accents are hot. In Marzipan's Answering Machine 17, he calls Marzipan as "Sharpdene," the "new girl," who has a Southern accent.