Two things...you can protect your writing by sending a copy to an attorney, etc. Tell them not to open it, but keep it in the envelope you sent it and post it through the US post office. That way you can prove by the date stamped on the envelope it is yours and when you wrote it. I also always put copyright on the pages and also if it is a manuscript of novel, I always make sure it is registered with the WGA, East or West.

What a profoundly should-never-be-asked question. First, no one in this business will risk stealing someone's material. Every letter you insert into a ms is copyrighted the instant the type hits the page, whether it be paper or electronic. And that's just the legal aspect. No one in this industry would even think about stealing anyone's work unless it was the most fantastic thing they had ever seen, and the likelihood of that is nil. Even then, the legal aspect would reign supreme.

There are three things you never do to an editor or publisher. The first is to say anything about how your work is copyright protected and don't you dare steal it. The second is to question their integrity regarding the work they do at any level and in any stage. The third is to ignore any small detail involving the format requirements. Not even a little bit. To do any one of these three things is to risk the wrath and exceptionally long term memories of editors and publishers alike. Any one of them is somewhat akin to pissing off your attorney before an important case or your surgeon before a delicate surgery.

Doing any of these things is also admission up front that you have no experience in the business of writing.

So, if you're looking for a compatible editor, that is something else and is legitimate. Most editors are personable folk in the writer's mind until the first edit comes in, then they're rotten SOBs for not recognizing the writer's immense skill at turn of phrase and incisive use of word. So, my question to you is, specifically what is it you are looking for? Otherwise the answer can only be generalized. You know, like search the lists of editors who are members of editor's associations. I've been working freelance for over 40 years and have never joined an association. Does that mean I'm not trustworthy or a bad editor or a fly-by-night operation looking to rip off the lowly writers who hire me? The best way to find that out is to ask those for whom I've done the job. The thing to understand from that is, there are lists of editors who are members of associations who have met their criteria, and there are freelance independents. If you look to an independent, ask him or her for references. Of course, it would be wise to do the same of association members, but at least with them there may be some recourse. Recourse for what, I don't know.

I found a great freelance editor with publishing house creds on LinkedIn a couple years ago. Her name is Tiffany Yates Martin and her company is Fox Print Editorial. I sent her a sample of the book I was working on and she reviewed it to be sure she wanted to work with me before she accepted my as a client. The first thing she did was recommend several books which changed the way I wrote. I ended up dropping the project after the third rewrite, but I learned a lot and my storytelling skills have significantly improved.

I don't think many true editors would steal your material. That risk, if it happened, would probably come from a stranger offering to help.
I edited magazines for very many years, ans have also helped a couple o9f writers edit their work for publishing. The last thing on my mind would be stealing it! In fact, an author friend who lives about 6 miles away has just given me her third book to pass my editor's eye over. (I helped with her second book to the extent of even giving her the plot and story line.) This third book involved quite a few pleasant evenings, sipping wine and discussing the book as it progressed.
I wish you luck.