1. This ignores the historical context of the Confederate flag. The flag was created before the Civil War and used as a symbol to represent a 'nation' that wanted to keep slavery as can be seen in the reasoning for secession behind every Confederate state.
2. The 13th Amendment does allow for slavery, see the bolded portion. It states that "Neither slavery nor involuntary servitude,
except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction." (
https://www.law.cornell.edu/constitution/amendmentxiii)
3. The Emancipation Proclamation freed slaves being held in specifically rebel states. Southern states that sided with the Union were still allowed to keep their slaves.
4. The North really didn't need the EP to gain more troops. Simply put, the industrial capacity of the Union states were so great that it outnumbered the South.