The first issue could be handled by registering for notifications with the NSUndoManager for Mac (sorry, I don’t have example code). Not sure about win32/linux The second could be handled with NSUndoManager declares (removeAllActionsWithTarget:), or by using a control array and creating a new textarea before you populate the text (but that seems like a major hack…)
It sounds like you’re running into some of the issues that prompted me to write TheBigUndo… it’s a commercial solution that I offer that would make this easy to handle.