If this 2 messages are MRM, you dont need to create 2 Msets, you can use the same.
In Mapping node you can change the CCSID just assigning a value , but you need to know what CCSID is (like 819, 1208 etc...).
You can use ESQL to change CSSID too.
Set OutputRoot.Properties.CSSID = 819;
3) WMB will not change CCSID or Enconding according to the target, WMB cant know what target is waiting for.
4) Yes, when you receive a message, WMB will take enconding and CCSID and overrides it. (You can see it at OutputRoot.Properties).