How to calculate SLA (weekends included)

This proposal employs eleven helper columns (D:N) which may be moved and/or hidden for aesthetic purposes.
Column D is populated using:

Formula:

=MIN(INDEX(L$5:L$11,MATCH(A24,K$5:K$11,0)),IF(WEEKDAY(B24,2)<=5,I$5,I$6)-MOD(B24,1))

Columns E:N are populated using:

Formula:

=IF(OR(SUM($D24:D24)=INDEX($L$5:$L$11,MATCH($A24,$K$5:$K$11,0)),ISNUMBER(MATCH(INT(SUM($B24,COLUMNS($A21:A21))),$H$13:$H$16,0))),0,MIN(INDEX($L$5:$L$11,MATCH($A24,$K$5:$K$11,0))-SUM($D24:D24),IF(WEEKDAY(SUM($B24,COLUMNS($A24:A24)),2)<=5,$I$5,$I$6)-SUM($D24:D24),IF(WEEKDAY(SUM($B24,COLUMNS($A24:A24)),2)<=5,$I$5-$H$5,$I$6-$H$6)))

The SLA Due column is populated using:

Formula:

=SUM(SUM(INT(B24),AGGREGATE(14,6,D$23:N$23/(D24:N24>0),1)),IF(WEEKDAY(SUM(INT(B24),AGGREGATE(14,6,D$23:N$23/(D24:N24>0),1)),2)<=5,H$5,H$6),INDEX(D24:N24,AGGREGATE(14,6,(COLUMN(D24:N24)-COLUMN(C24))/(D24:N24>0),1)))

Let us know if you have any questions.