importing a file with variable columns

I would use approach 1 or 2.  The deciding factor is the size of the file, number of records.

Since you know the first n columns are fixed to specific dimensions, and the remaining columns are specific to accounts, assuming column header = account name, this is pretty easy to pivot, something like this:

Function PivotAccounts(inputTable As DataTable) As DataTable
    ' Prepare output table
    Dim outputTable As New DataTable()
    outputTable.Columns.Add("time", GetType(String))
    outputTable.Columns.Add("entity", GetType(String))
    outputTable.Columns.Add("ud1", GetType(String))
    outputTable.Columns.Add("ud2", GetType(String))
    outputTable.Columns.Add("ud3", GetType(String))
    outputTable.Columns.Add("account", GetType(String))
    outputTable.Columns.Add("amount", GetType(Decimal))

    ' Validate column count
    If inputTable.Columns.Count < 7 Then
        Throw New InvalidOperationException("Input table must have at least 7 columns.")
    End If

    ' Fixed column indexes
    Dim fixedColCount As Integer = 5
    Dim lastColIndex As Integer = inputTable.Columns.Count - 1

    ' Iterate through each row
    For Each row As DataRow In inputTable.Rows
        ' Extract fixed values once per row
        Dim timeVal As String = row(0).ToString()
        Dim entityVal As String = row(1).ToString()
        Dim ud1Val As String = row(2).ToString()
        Dim ud2Val As String = row(3).ToString()
        Dim ud3Val As String = row(4).ToString()

        ' Iterate over the account columns in the current row
        For i As Integer = fixedColCount To lastColIndex - 1
            Dim accountName As String = inputTable.Columns(i).ColumnName
            Dim rawValue As Object = row(i)

            If Not IsDBNull(rawValue) AndAlso Decimal.TryParse(rawValue.ToString(), Nothing) Then
                Dim amountVal As Decimal = Convert.ToDecimal(rawValue)

                ' create a record for each account's amount
                If amountVal <> 0D Then
                    Dim outputRow As DataRow = outputTable.NewRow()
                    outputRow("time") = timeVal
                    outputRow("entity") = entityVal
                    outputRow("ud1") = ud1Val
                    outputRow("ud2") = ud2Val
                    outputRow("ud3") = ud3Val
                    outputRow("account") = accountName
                    outputRow("amount") = amountVal
                    outputTable.Rows.Add(outputRow)
                End If
            End If
        Next
    Next

    Return outputTable
End Function