WTD for same week last year

None of your formulas use the week number. It will be much simpler if you took the week number, and then just sum up all the days in that week until the current day of the week - for both years.

// To do this properly, you have to
// create some columns in your Dates table:
// YearOfWeek - an int, the year the week (to which
//              the day belongs to)
// YearWeekID   - an int, the id (consecutive from 1..52)
//                of the week in the year
// DayNumberInWeek - an int, the day (1,2,...,7) in
//                   the week
// Once you have these, you can write:

[Measure WTD LY] =
var __currentYear = SELECTEDVALUE( Dates[YearOfWeek] )
var __currentWeek = SELECTEDVALUE( Dates[YearWeekID] )
var __lastDayNumberInWeek = MAX( Dates[DayNumberInWeek] )
var __lastYear = __currentYear - 1
var __prevWeek = __currentWeek
var __output =
    CALCULATE(
        [Sales],
        Dates[YearOfWeek] = __lastYear,
        Dates[YearWeekID] = __prevWeek,
        Dates[DayNumberInWeek] <= __lastDayNumberInWeek,
        ALL( Dates )
    )
return
    __output

Bear in mind that you'll have to give a special treatment to the first week in the second year or/and the last week in the last year since the first week in the first year in your Dates table might not have all days and the last week in your last year may not have all days. These are the edge cases that you have to tackle reasonably.