I speak Spanish and I didn't really understand the difference between B and C, anyway I considered that C was a palindrome in itself, like what I did, I chose different timbres for both voices for a matter of hearing comfort to all those who go through the works, so they can differentiate the lines well, in reality they have the same timbre in the score. About the echo I don't understand well, a palindrome doesn't necessarily have to be inverted with a program or DAW, it is simply played by the instrumentalists from back to front
About the 50s thing, it's a mix error that I tried to fix but I couldn't :( I think it was because I used a bad plugin, I'm just learning to mix this year and there are certain things that I can't fix yet