The problem is that treason trials went too far. Prison terms and permanent loss of status (no voting, limited rights, restrictions on travel and speech) for those in senior positions would have been fair. Going as far down as they did was inappropriate, any prosecutions on a more junior level should have been under the rubric of war crimes, such as executing black POWs, killing civilians deliberately and so forth.
Additionally, having all former Confederates sign an oath of allegiance would have been helpful - those who kept fighting, joined the KKK, or abused blacks in contravention to established law would be opening themselves up to long rpison terms, confiscation of assets, or even loss of citizenship. Loss of voting rights for officers, for a period based on their rank (say 5 years for junior officers, 10 for mid grade, and 20 for colonel and above) would have allowed blacks and poor whites to build a political structure that would prevent the prewar planters from reestablishing political control.
As far as the Europeans go, what hypocrites. In the 1860s get convicted of treason in the UK, France, or Germany or attempt to lead an armed rebellion and you would not just get a slap on the wrist. Not that they were wrong in that the USA went too far, but to take the moral high ground...really!