I believe you. Washington DC has been ran by influence sellers and traders for a long time.
As Lincoln said, Chase got the presidential bug, and once you get it, you don’t lose it. His circuit happened to include Virginia, where he could go down and try cases. The last thing he wanted to do was alienate the people in the returning states.
Speculative. Might be correct, but an opinion is not proof. It is also possible that Chase did not believe secession was treason.
Also, the Johnson Administration wanted to build up its support among Southern Democrats and trying Davis wasn't the way to do it.
Also likely true.
Chase had been a Whig, a member of the Liberty Party, a Free Soiler and a Democrat before becoming a Republican. While Chief Justice he switched over to the Democrats (with an interval as a Liberal Republican) in hopes of getting the Democratic nomination for president. Politics surely had much to do with his political opportunities which he now saw as more in the Democrat than the Republican Party.
More complications: Radical Republican Thaddeus Stevens actually wanted to defend Davis. He wanted secession to have been legal and happened, so that the South could now be treated as a conquered province. Chase believed that the ban on secessionists holding office was "punishment" enough for Davis. He did not believe that secession was legal. He just didn't want a trial. Andrew Johnson's blanket pardons (though Davis didn't want one) ended the possibility of a trial.