To paraphrase JFK, ask not editors to publish what you write, write what editors want to publish.
To have any success, you have to pitch a commissioning editor with more than just an idea; it has to be a properly researched idea, which matches the interests of the publication's target readership. So, before you even think of pitching anything, make sure you are familiar with the particular market. Read the magazines you want to submit to. Work out the kind of articles they publish; their approach to the subject, their tone, their lengths. Ideally, read more than one issue, as that'll give you a better idea of the "closed" slots filled by the same contributors every time (ie, by in-house staff or regular freelancers) and the "open" slots definitely provided by outside contributions. Check out the publisher's website -- not the magazine's website -- and look for any sales/marketing information you can as that will give you an idea of the publication's intended readership.
Then, you "just" have to come up with an appropriate idea that you think will match the publication, and deliver it in such a way that the editor says yes. (Easy? Well, no... otherwise, everyone would be doing it.)
Regarding your specific points: if your pitch is sufficiently interesting and is presented professionally, then a professional publication should take you seriously. Never, ever assume that they won't bother with you simply because you've never written for them before. (After all, if they only ever used people who had already written for them, how would they have started the magazine in the first place?)
Remember: there is no copyright on ideas, only specific representations of ideas.
Before I went freelance, I had some experience as a commissioning editor and can tell you that, in such a position, the last thing you want is any trouble. To be honest, I think you're being slightly paranoid. Any publisher or publication which took unsolicited ideas and gave them to other people (such as in-house staff) to write up would very quickly get a bad reputation in the industry. Trust me, it's highly unlikely to happen.
When it comes to pitches vs articles, the bottom line is whatever the publication prefers.
However... in my experience, the vast majority of editors are busy people who much prefer to read pitches, and the more succinct these are the better. In fact, I'd go as far as saying that writing an article before you've been commissioned to do so is the mark of an amateur -- why put in all the work without the guarantee of being paid for it? Also, if they don't take the article, it's a lot of wasted effort on your part, or the article will require a lot more work to reshape it to the house-style of another publication.
Any other advice?
Always find out the name of the person you're sending the pitch to. NEVER just send it off "Dear Sir/Madam" or "Dear Editor"; if nothing else, it shows a lack of research on your part. Also; if a publication has a deputy or assistant editor, my experience is to pitch them rather than the editor; editors tend to delegate dealing with unsolicited submissions if they can. If you're pitch is good, it will be forwarded up the editorial chain.
Regardless of the title and its subject, remember that commissioning editors are looking for appropriate copy; editorial content that will be of interest to their readers (old and new alike). So, you have to ensure your article idea fits their demographic. Or, to put it another way, your pitch has to clearly answer: Why Them?
Remember that most magazines work several months in advance: monthly publications will have a pretty good idea what they'll be publishing six months from now, so always focus on the future. You must be able to answer their question: Why Now?
Most importantly, you need to show that you're not just capable of writing the article but are also the best-suited person to do so. This could mean you have the necessary knowledge; more likely, you know it's because you'll have access to the most authoritative experts on the subject; you've seen the particular "angle" on a subject that no one else has, and can back it up in an interesting way. This is probably the most difficult of what I call the Three Whys, but it's also the most important. You need to have a convincing answer to their question: Why You?
And all of this within, at most, a hundred or so words. Yes, keep it as sharp and brief as you can.
Finally, remember this: They are likely to say "No." Don't despair. Don't think of it as the end of the conversation (unless they're mentioning restraining orders, of course!); it's the beginning of one, between professionals. Amateurs quickly give up, professionals are persistent; you are more likely to get a sale if you have built up some kind of relationship with the commissioning editor. Just don't be scary.
Hope this is of some help.