Helping one's mother run her business

Q:

1. I currently live with my mother who runs a business from home, many a times after I return from work she requires that I assist her with her business and this takes up my time for ibaadat and serving deen. Am I sinful in refusing to assist.

Note it is not household chorus but business related.

2. I intend getting married and my mother demands that I should come by every night to assist her with her business, am I sinful if I refuse in this regard as it will take strain on the marriage.

Offer and acceptance of a business transaction through actions

Q: I recently read that a sale takes place with a verbal offer and acceptance from both parties. However, in today’s times, we notice that at the time of purchase, customers merely place the money on the counter while the cashiers hand over the goods without any words being exchanged.  Will the sale be valid in these cases as no verbal offer and acceptance is apparently found?

Will preventing someone from walking in front of one while performing salaah be considered amal-e-katheer?

Q: It is written in Behishti Gohar that it is allowed to prevent a person from crossing in front of him while he is praying. However, if 'عمل کثیر' happens, then the prayer will become fasid.

What will be considered عمل کثیر ? What should the musalli do to prevent the person from crossing him?

Intention when making qadha salaah

Q: Regarding qadha salaah (missed prayers):

What is the full intention to make before one performs a qadha salaah (Note: the intention for ample salaah missed not just a single salaah). I am fearful that if my intention is incorrect my qadha salaah wont be accepted. Many books have the intention for only missing one prayer. 

To my knowledge the intention is "O Allah, I am reading the 1st fajr of those that I have missed" but I am unsure if it is correct.