I think you can approach this with a loop instead of using HTML in AMPScript variables.  I'd recommend using a loop based on `flag_weekstart`.  Something like this:

    %%[
    
    var @Total5, @Total4, @Total3, @Total2, @Total1, @flag_weekstart, @today
    set @today = now(1)
    set @Total5 = AttributeValue("5Total")
    set @Total4 = AttributeValue("4Total")
    set @Total3 = AttributeValue("3Total")
    set @Total2 = AttributeValue("2Total")
    set @Total1 = AttributeValue("1Total")
    set @flag_weekstart = AttributeValue("flag_weekstart")
    
    /* for testing */
    set @Total5 = "123"
    set @Total4 = "234"
    set @Total3 = "456"
    set @Total2 = "789"
    set @Total1 = "012"
    set @flag_weekstart = 4
    
    ]%%
    
    <table border="1">
    
      %%[
    
      /* output headings */
    
      for @i = @flag_weekstart downto 1 do
    
        if @i == @flag_weekstart then
          output(concat("<tr>"))
        endif
    
        set @headingDate = formatDate(dateadd(@today,multiply(-7,subtract(@i,1)),"D"),"YYYY-MM-DD")
        
        ]%%
          <td>Week of %%=v(@headingDate)=%%</td>
        %%[
    
        if @i == 1 then
          output(concat("</tr>"))
        endif
    
      endif
    
      next @i
    
      /* output row */
      for @i = @flag_weekstart downto 1 do
    
        if @i == @flag_weekstart then
          output(concat("<tr>"))
        endif
    
        if @i == 1 then
        ]%%
          <td>%%=v(@Total1)=%%</td>
        %%[
        endif
        if @i == 2 then
        ]%%
          <td>%%=v(@Total2)=%%</td>
        %%[
        endif
        if @i == 3 then
        ]%%
          <td>%%=v(@Total3)=%%</td>
        %%[
        endif
        if @i == 4 then
        ]%%
          <td>%%=v(@Total4)=%%</td>
        %%[
        endif
        if @i == 5 then
        ]%%
          <td>%%=v(@Total5)=%%</td>
        %%[
        endif
    
        if @i == 1 then
          output(concat("</tr>"))
        endif
    
      endif
    
      next @i
    
      ]%%
    
    </table>

**Output**

    +------------------+------------------+------------------+------------------+ 
    |Week of 2017-09-19|Week of 2017-09-26|Week of 2017-10-03|Week of 2017-10-10|
    +------------------+------------------+------------------+------------------+
    |234               |456               |789               |012               |
    +------------------+------------------+------------------+------------------+