Hi Sara,
Welcome to the world of mystery shopping. I can tell by your expansive analysis and thoughtful questions that you are a motivated shopper with excellent writing skills. Take advantage of as many courses and conferences as you can handle. Save time, effort, and money by learning the right way from the beginning and avoiding other shoppers' mistakes. There are many companies which only require multiple choice responses or short narratives with fine dining and hotels being the most notable exceptions as these typically require lengthy narratives. While I've never seen an "Only Gold May Apply" requirement, certifications provide a baseline to the MSC and can open doors to assignments never posted on the job boards.
"We don't know what we don't know." The value of picking up even one new skill will boost your confidence to take on more challenging assignments. Attending a mystery shopping conference introduces you to owners, schedulers, editors, and other shoppers and is a great networking opportunity. Certifications can and do open doors for you. I was contacted by a MSC directly as a result of my having passed an auditor's certification exam at a MSPA conference resulting in substantial pay for ongoing assignments at multiple locations. In addition to MSPA, also check the excellent courses and conferences sponsored by Independent Mystery Shoppers Coalition at imscinfo.com and Video Shopping Pros at videoshoppingpros.com.
While your planning for courses and programs, keep in mind the basics. Apply to as many companies as possible and build your reputation by being a responsible reliable shopper. This means completing the shop according to the guidelines by shopping at the correct location on the assigned date and timeframe, filing reports timely, and checking back to see if there are editor's comments requiring further clarification or documentation.