Average Values with multiple criteria

Hi, croine 
I got your result:
There are some things that could be optimized but if you need it to work in the first place, it should be all you need:
I started with creating some "Ranking" columns in order to evaluate 7 ABOVE rows, I used simply Index Column in Power Query and worked with it like that.
Then I added two columns to check, whether the column is empty or not (This could be optimized later)
Finally, two more columns that actually fill the missing info based on your criteria (you can't fill existing columns in DAX)

Here is the code:

DailyAverageBlankCheck = 
var BlankCheck = IF(ISBLANK('Table'[Daily Average]),0,1)

return BlankCheck

FixedHourBlankCheck = 
var BlankCheck = IF(ISBLANK('Table'[Fixed hour]),0,1)

return BlankCheck

DailyAverageFixed = 
var currentRank = 'Table'[Index]
var sevenRowsAbove = FILTER('Table', currentRank >= 'Table'[Index])
var TableForAverage = TOPN(8,sevenRowsAbove,'Table'[Index])
var Top7Check = CALCULATE(SUM('Table'[DailyAverageBlankCheck]),TOPN(7,sevenRowsAbove,'Table'[Index]))
var DailyAverage = CALCULATE(AVERAGEX('Table','Table'[Daily Average]),FILTER('Table','Table'[DailyAverageBlankCheck]=1))

var Result = SWITCH(TRUE(),
    NOT(ISBLANK('Table'[Daily Average])), 'Table'[Daily Average],
    'Table'[DailyAverageBlankCheck]=0 && Top7Check=6, AVERAGEX(TableForAverage,'Table'[Daily Average]),
    DailyAverage)

return Result

FixedHourFixed = 
var currentRank = 'Table'[Index]
var sevenRowsAbove = FILTER('Table', currentRank >= 'Table'[Index])
var TableForAverage = TOPN(8,sevenRowsAbove,'Table'[Index])
var Top7Check = CALCULATE(SUM('Table'[FixedHourBlankCheck]),TOPN(7,sevenRowsAbove,'Table'[Index]))
var HoursAverage = CALCULATE(AVERAGEX('Table','Table'[Fixed hour]),FILTER('Table','Table'[FixedHourBlankCheck]=1))

var Result = SWITCH(TRUE(),
    NOT(ISBLANK('Table'[Fixed hour])), 'Table'[Fixed hour],
    'Table'[FixedHourBlankCheck]=0 && Top7Check=6, AVERAGEX(TableForAverage,'Table'[Fixed hour]),
    HoursAverage)

return Result

 
Sample data screen: