These are all good questions.
It seems to me that the most important factor for you to consider is the attitude that your clients will take. Never mind who owns the copyright. If a client believes (rightly or wrongly) that they are the copyright owners, they will feel justifiably aggrieved if you go ahead with the posting - especially if you do so without their permission.
Hiding the clients' names doesn't really change that. If the client recognises the content as theirs, the damage will be done.
The obvious solution would be to contact the clients, explain the situation, and ask their permission to post the articles. Point out that you will give the client an acknowledgement, and include the client's copyright notice if there is a reasonable possibility that they are entitled to that. Be sure to tell them that you are doing this only to show a portfolio of your work, not to compete with the client in any way.
I doubt very much that any of your clients will object to that.
As far as duplicate content is concerned, that's an overblown issue. Having more than one copy of a given piece of text is much less of a problem than many people suppose. However, if the client is likely to be at all concerned about that, you can offer to add a noindex attribute to your copies, to ensure that they won't show up in the search engines. That will satisfy the client's concerns, and it won't do you any harm either. After all, your aim is for your site to be seen by your potential clients - people who will be searching for terms like "freelance writer" or "copywriting in <name of city>" - not those searching specifically for the articles in your portfolio.