#1 - #3
#1 is fixed in the latest version. Yes the dataset -a is not working properly and we will look into it. Can you use StringArray for now? e.g.
StringArray sa = {1,2,A,B,A};
sa.copyto(Col(A));
#4
Can you highlight the data in the worksheet and click the Mask Range icon at the bottom toolbar. Then activate one graph, and click the Hide/Show masked points icon at the bottom toolbar. This is global setting and hide the masked points for all graphs in the project. See if it works for you.

James