I'm going through musuo mode and I keep trying all the characters, but I'm starting to get B ranks on the mission. Should I just be putting all my EXP and game time into one or two characters or is it okay to use them all?

If i do start using only one or two, can I just level up different weapon masteries on them and switch weapons instead for variety?

Semi related, but are the attack power of weapons all the same, but with different gem slots? or at there just better weapons and how do I get them?

Any help or advice would be appreciated thanks!