1. There is a "Action" called "Set Variable" which could be used to clear variables e.g. OnActivateSheet.
It works generally fine on variables, but if you use the Calendar-object it does seem like that overrides this action and you cannot change a selected date.
One possibility would be to run this in steps so e.g. vToDate is set to vCalToDate (connected with the Calendar object) and then you clean out vToDate.
2. In the Charts > General Tab you have the standard "Error Messages", where you can switch to a custom message.
3. You can work with regular lines breaks within quotes. E.g.
='Here is my first line
Here comes my third line
and some more...'