Amazing post, Dave, and definitely an important topic. My first book I used one of the larger “editorial agencies” you mentioned. The quality wasn’t bad but there was no direct contact with the editor. Just ship it to them and they ship it back. I’ve tried out several editors now and found a very good one who did a great job on my list book. Once you find a good editor, stick with him/her and build a relationship if you can. Thanks for the list provided and this editors test is a great addition.
Look at the front and back of the next page for acknowledgements. If an author worked closely with an editor throughout the writing process, they may choose to include their name in the acknowledgements page. The acknowledgements page is usually a few lines of text centered on a blank page after the copyright information. Read it to see if the editor is mentioned.
I say “most of your desired criteria” because it’s rare to find an editor who will meet all your criteria. For instance, you may have to pay a few hundred to a few thousand dollars more for your top pick. Or, you may find someone at your precise price point, but their experience isn’t quite what you’d like it to be. You must be the one to assess what trade-offs you’re willing to make.
There may be provision to have a call to discuss the work, or it may purely be the delivery of a structural report or a line edit or proofread manuscript. I've worked with professional editors for the last ten years and have never had a call to discuss, but if you're the type of person who wants that, then make sure you put that into the contract upfront along with anything else you are concerned about.
Obviously there is a big difference. I actually started with screenplays (with some mild success) partly due to hearing that as long as you have a good story, the grammatical errors won’t be a big deal. Well, that was not true. Maybe if a producer liked the story or I was already established but every contest I entered always came back with feedback about the grammatical errors. Even if I won or placed as a finalist, I always received critique on that. Very little is spelling error, it was more about structure, etc.