As you've realised symbols have their own API which can be/is used when you need to access any timelines within that particular symbol. When you want to reference the symbol you are not constrained to use "symbolInstance" you can use any naming convention you want
var scrollTextSymbol = hypeDocument.getSymbolInstanceById('mySymbolInstance');
The "minor" error you're getting is because of the first declaration of "time" you are looking for the timeline using "hypeDocument" when you should be using the "symbolInstance" declaration.
var symbolInstance = hypeDocument.getSymbolInstanceById('mySymbolInstance');
var time = symbolInstance.currentTimeInTimelineNamed('txt_scroll_2');
would be the correct syntax.
Here, in your document, "txt_scroll_2" as a timeline only exists within the symbol.