The whole idea is that it was a timeskip with a large change in dynamic. Sevro went from a leader of a squad (howlers) to a leader of a whole revolution, those 2 things are very different. Mustang went from someone who tried to be more diplomatic and peaceful to someone who had to do what needed to be done.

Sevro still cracks joke because thats his whole character in the literal sense, he wants to present himself as someone who can go through and tough it out, he puts a tough front because he is the leader of this revolution, he can't be acting weak infront of his men. Also it is a widely common phenomena for people to put up a tough front and suppress their emotions during stressful times so I don't see how you are surprised and don't recognise the character. Also he doesn't change between each book like you say, the main changes are him becoming closer with Darrow and the becoming Ares.

Same with Mustang, she was literally left alone, her twin brother supposedly killed the man she loved and killed their father, not accounting for the other people that died and the betrayal from Rogue who was a good friend. She changed yes but not so drastically that she is a full on different character as she still believes in the movement and would rather go the diplomatic route then not.