wrong grand total using if statements

It's actually very easy to do.... You just have to calculate the measure row by row over all the combinations of (week_ending_on, workername), which means you have to write the measure using SUMX. Here's something to get you started (but you have to learn more about DAX, I guess):

[Measure] =
SUMX(
    summarize(
        T[week_ending_on],
        T[workername]
    ),
    // Here, since SUMX is an iterator
    // you'll know that you have a row context
    // in which only one worker and one
    // week_ending_on are visible. Just write
    // the measure with this in mind and it
    // should work... Please learn from https://dax.guide/calculate
    // how CALCULATE interacts with row contexts.
    <your_measure>
)

Anonymous 

Create a new variable based on the one you already have. See it at work in the attached file.

new applied hours TOTAL = 
SUMX (
    SUMMARIZE ( 'table', 'table'[week_ending_on], 'table'[workername] ),
    [new applied hours]
)