Perhaps you could use a query to calculate the count of weekdays. Something like:
SELECT Count(YourTableName.FieldNameToCount) AS NumOfWDays
FROM YourTableName
WHERE (((YourTableName.YourDateFieldName) Between [Start Date] And Date()) AND ((Weekday([YourDateFieldName]))<>1 And...
Not sure that "MUST" is quite right. I thought that expressions without them would still work if the field name had NO spaces in it. I would agree that they are advisable though.
Perhaps you could set the sub form as Not enabled. Then put your validation code in the Main forms Before Update event and in the Main forms On Current event