Welcome!
I've been doing this for just shy of two years, so there are folks around with a lot more experience than I have, but I think I can answer your questions reasonably well.
* Everybody has a different way of making this work for them - the vast majority of shoppers only do this part time. I work full time and do this on the side and am able to bring in over $1,000 a month to my family. However, I rarely do shops that pay less than $30 or $40. You do not get directly reimbursed for gas, but you can get bonuses for longer trips, if the company can't find somebody willing to do a job at an asking price. (We are operating in system of raw capitalism.) Finding higher paying shops is a matter of finding the right companies, being willing to drive a little sometimes, and learning the patters for when companies do and do not bonus shops. I also do video shopping - which can pay very well. (I did three shops yesterday for a total of $240.) It can take some time to establish yourself.
* This can work for folks in cities and in more rural areas. The biggest key is whether or not you are willing/able to drive to the money. I drove 90 miles for one of my shops yesterday and, overall, spent more time driving than I did combined writing up reports and doing my visits. There tend to be a lot more opportunities in cities, but there also are a lot more shoppers, keeping fees relatively low. MSC's can be desperate to fill shops in more rural areas and will often offer very large amounts of money for simple shops.
* Another strategy you might consider: There are a lot of reimbursement-only shops. Instead of striving to bring in a paycheck, you can strive to be reimbursed for expenses that you already have: Groceries, oil changes, delivery pizza, trips to the movies with kids, etc.
Hard work builds character and homework is good for your soul.