It does make sense that KHTML was a compromise for Apple as you say. It's interesting that that was the motivation there, thanks for sharing that! About Opera,

It does make sense that KHTML was a compromise for Apple as you say. It's interesting that that was the motivation there, thanks for sharing that! About Opera, a comment: Opera definitely has real commercial viability. Unlike the other three you mention, Opera has had to be profitable all its life, and from the Opera browser itself (not subsidized by other products). Today Opera makes a lot of money from its mobile browser (but it is in danger from WebKit now).