1) You should be able to right click that attribute, Create Set. Then Edit Set. Select whichever members you want involved in that filter. Then Show Quick Filter. That Quick Filter now only contains the members you chose from the attribute you selected from the SSAS hierarchy. I have done this with Quarters from the Date Dimension.
2) I would like to know how to do this one too, not sure if it's a limitation of SSAS/Tableau or what. My next thing to try is to create a blank member in the cube (a value of null) and somehow see if we can the text to be white/transparent just for that member. I don't think it's going to work.