Hi shellmis and welcome! There is a list of some 200+ companies in the first post of the first thread of the Mystery Shopping Company Discussion section of this forum. You now have your feet wet, so have some sense of what mystery shopping is about. If you plan to pursue it, getting signed up with many companies is the next step. Not all will have work in your area, different ones may have work after the beginning of the year when clients tend to move from company to company, some are so focused on a small geographic area that unless you are right there, you will never see much (if any) work with them.
To your specific questions: 1) For merchandising you really should check the Narms.com website and sign up there. To a great extent, getting started in merchandising seems to be a matter of luck. I know of a number of shoppers who have turned to the merchandising work as their primary employment because it tends to be more structured and systematic, with mystery shops added along the way as it may be convenient. I gather that the better demo work and merchandising is done through Narms companies rather than the bits and pieces of merchandising that arise from companies that focus on mystery shopping.
2) Best companies for Mystery Shopping is like asking which child is your favorite. We all have preferences that are based on payment cycle, payment size, ease to work with, variety and interest of the jobs, etc. etc. You really do need to sign up, perform a job for the new company and see how well they mesh with you and your needs. There is discussion here about companies that are NOT keeping their end of the bargain, so do a little research using the "Search" feature before deciding whether or not to work with a company. One person unhappy does not make a bad company, a chorus of "me too!" should make you wary.
3) There are lots of threads throughout this forum about whether certification is worth it. The only "certification" to even consider is that sold by the MSPA. My personal opinion is that it is worthless, though others in other markets feel it has opened doors for them they could not have opened otherwise. Who can prove anything one way or the other? $15 is not a lot to spend to buy Silver Certification, but then again you are working to MAKE money, not to be parted with it.