the number of hours is divided by 24 only when the X2 value is a decimal number of hours, if you have an actual time value in X2 like 15:00 you should remove the /24s, so your last example will work with this formula

=WORKDAY(M2,CEILING((X2+MOD(M2,1)-BusinessStart)/(BusinessEnd-BusinessStart),1)-1,HLDY)+MOD(M2,1)+X2-CEILING(MOD(M2,1)+X2-BusinessStart,BusinessEnd-BusinessStart)+BusinessEnd-BusinessStart

.....but you want that to work even if M2 is outside working hours?