1. I don't see why the 90-minute move should make much difference. Shopping in S. CA is like route shopping; No matter where you live, you will be on the road a lot.
2. Shopping with a two year old would be a challenge, inefficient, and most likely uncomfortable for the child. You could not do revealed audit with a child. What's he going to do, sit in the car for 30 - 60 minutes and wait while you conduct the audit? Good luck with that. However, it might be enjoyable for him to travel with you as he gets a few years older.
3. If you enjoy phone shopping, go for it! There are not as many phone shops as written, but it will not be uncomfortable for you or your son. There are some shops that allow you to bring children, but I imagine a two year old child would likely distract you from the shops. A child that young requires constant supervision.
4. No, not unless you have difficulty with organization. Mystery shopping conventions cost a lot relative to the benefits you may get, unless you are a newbie shopper. Expenses include at least three days' missed work, possibly hotel expenses, increased meal expenses to eat out, and travel expenses.