Handling of memorystream with streamwriter efficiency
It looks like you are wasting your efforts on premature optimization. Which also results in a quite ugly code.

I think you should separate your grid-filling logic from csv export. There should be no memory streams, steam writers, etc. in your first method. Also notice that you have code duplication in it, so you might want to extract label creation logic to separate method. Ideally you'd want to remove this logic all together and use WPF ListView (or ItemsControl) with data-binding instead.

As for your second method - you do not need any buffers. Just iterate through your data, create strings and write them directly to your FileStream. You might also want to encapsulate this logic in some separate class and perform it in the background thread so your UI does not freeze for large data.