You draw distinctions that use idiosyncratic definitions of some fairly standard terms. For example, Chicago defines the terms “manuscript editing,” “line editing,” and “copyediting” as synonymous with each other, and none of them as synonymous with “mechanical editing,” although that is one element of them. Certainly, I agree that an author has a right to expect more than merely mechanical editing unless that is specifically what they have requested. However, I cannot agree that the only route into competent and appropriate freelance editing is internal employment within a publishing house, and that the only clients who should be counted as a relevant part of a freelance editor’s curriculum vitae are those who are traditionally published.

Freelance editors normally do not double as literary agents. Not only is it not our area of expertise, but it can actually lead to ethical quandaries. There have, unfortunately, been cases of literary agents saying that potential clients’ work “has potential” but needs to be edited (by them or their employee), for an exorbidant fee. It would also be possible for editors to give the impression that they are guaranteeing publication of a project, when that is something no one but a publisher can do. Many professional organizations forbid their members to combine these two roles, to avoid a possible conflict of interest.

Working with a professional editor can be one of the most rewarding experiences in an author’s life. You’ll probably learn more about the craft of writing — plot, characterization, dialogue, worldbuilding… — than if you took an creative writing course. However, as is always the case in creative endeavours, critique can be hard to accept. This is why you should seek an editor who’s not only experienced in their field, but whom you’d feel comfortable receiving feedback from. In the words of one of our Reedsy editors:

I got in touch with Ebook Launch lately to sort out a cover for my second book, and I cannot recommend them enough. The process was quick and easy, and the effort put into capturing the mood of my story was apparent from the first sketch. Their service was friendly and accommodating, and they were open to making or reverting changes to my covers without any trouble. I appreciate the hard work they put in on my behalf, and wholly recommend their services.
That said, I don’t recommend going to publication, particularly if your book is intended as a revenue-generating venture or as a vehicle of professional promotion, without making sure that someone with an eye for detail has gone through the entire manuscript and ruthlessly noted every typo, every spelling error, every misplaced comma, and every ambiguous sentence.

That’s a great point about other disciplines. Why should writing require some outside arbiter? Is it because these other disciplines consist in crafts that require significant training, whereas anyone who’s literate can start writing a book at any time (and therefore, the threshold for entering the medium is much lower)? I do hope you’re right about learning self-editing skills. I actually just made a YouTube video about how to self-edit:
2. Copyeditors aren’t the last line of defense when it comes to a book’s mistakes. Formatting, which is done after an editor has done their job, can lead to unintended errors being introduced into the text. That’s why it’s important to hire proofreaders to comb through edited text *after* the book has been formatted for print or digital. Proofreaders are your last line of defense—but they’re human too.