Calculate open credit; avoid blank values

Hi, 

I am trying to create an accounts receivables report and I want one list to show data about the amount of open credit of various customers, preferably in an area chart. My data look like this:

Basically I want to calculate a running total of the value of invoices, that have been created (invoice date <= selected date) and not yet paid (payment date > selected date or there is yet no payment date).

Best I could do so far is this: 

Open rec =
CALCULATE (
    [Amount],
    FILTER (
        ALLSELECTED ( 'Fact table' ),
        'Fact table'[Customer] = MAX ( 'Fact table'[Customer] )
            && 'Fact table'[Invoice date] <= MAX ( 'Calendar'[Dates] )
            && 'Fact table'[Payment date] > MAX ( 'Calendar'[Dates] )
    )
)

The problem is that the result should look like this 

 but it looks like this with missing values:

So I have three problems/questions:

1) What can I do to fill the blank values with the last non blank value?

2) This sample data include only paid invoices but in real data the Payment date could be blank meaning the invoice is still not paid and this should be reflected in the measure.

3) Can I create two measures - one for receivables due (simply not past the due date) and one for receivables overdue without using a virtual table? I would love to use it in a stacked area chart instead in a normal one.

Sample file attached: File 

Thank you for helping