Even assuming arguendo secession was unlawful, if Davis had a good faith belief that secession was lawful, there could be no criminal intent and, therefore, no crime of treason.
IMHO, no to both concerns. Even today, the lack of knowledge regarding the law is no defense. As an aside, for something to be illegal there must be a law against it, and as of yet, no one has ever pointed to a US law (or the Constitution) that prohibited secession.
In Justice Chase's Texas v White decision, he maintained the position held by Justice Story and by Lincoln, that the Constitution incorporated the "perpetual" nature of the union (overlooking the fact that a word used 5 times in the Articles was omitted, and the fact that the states seceded from the Articles). In that decision, Chase held that the state of Texas had never left the union, which renders his ruling on the payment of bond the reverse of what would be the case if Texas had not left (Justice Grier, author of the majority opinion in the Prize Cases, destroys Chase's arguments in his dissent in Texas v. White).
But back to treason. According to US code (those laws made pursuant to the Constitution) treason was defined thusly in 1790: Section 1. Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That if any person or persons, owing allegiance to the United States of America, shall levy war against them, or shall adhere to their enemies, giving them aid and comfort within the United States or elsewhere, and shall be thereof convicted, on confession in open court, or on the testimony of two witnesses to the same overt act of the treason whereof he or they shall stand indicted, such person or persons shall be adjudged guilty of treason against the United States, and shall suffer death. An Act for the Punishment of Certain Crimes against the United States (1 Stat. 112, 1790)
And this by Justice James Wilson ("Of Crimes Immediately against the Community, Lectures on Law" in 1791):
[W]ho may commit treason against the United States? To this the answer is--those who owe obedience to their authority. But still another question rises before us--who are they that owe obedience to that authority? I answer-- those who receive protection from it. ... "The Confederacy did not owe allegiance to the union, nor did they receive protection from it - the Union attacked the Confederacy.A traitor is hostile to his country"