Filter column only if variable is not blank

I figured it out.
Instead of the "switch values=a and values<>b, a1, values=b and values<>a, b1, else blank" code, i needed to do "if single value selected, then switch value=a, a1, value=b, b1; else blank"
And instead of "calculate column, if not isblank, filter", i needed to do "if isblank, column; else calculate column, filter"

Here's the code:

Actual Sell Thru (Channel) =
VAR EquivalentStoresSegment = 
    IF(COUNTROWS(VALUES('Product'[Is Retail Or Commercial SKU]))=1,
        SWITCH(TRUE(),
            VALUES('Product'[Is Retail Or Commercial SKU]) = "Retail", 
                "Stores Consumer",
            VALUES('Product'[Is Retail Or Commercial SKU]) = "Commercial" ,
                "Stores Commercial"
        ),
        BLANK()
    )
RETURN
IF(ISBLANK(EquivalentStoresSegment),
    [Actuals Sell Thru (Units)],
    CALCULATE([Actuals Sell Thru (Units)],
        REMOVEFILTERS('Product'[Is Retail Or Commercial SKU]),
        KEEPFILTERS('Business'[Stores Surface Segment]=EquivalentStoresSegment)
    )
)