“treason in 1860 can only be determined by prior law as developed up to that point in time.”
Article III Section 3 of the Constitution of the United States “Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort.”
Is that prior enough for this discussion?
Under your understanding of the language, every party to ever wage war with the US, from Mexico to Japan, has somehow committed "treason". Clearly, one has to be a US citizen and subject to US constitution jurisdiction to be able to commit treason, and the member states of the CSA had formalized their secession from the union prior the event that I am sure will insist began the war. So, unless "attempting secession" is also mentioned, you've got nothing.