How to redesign a client site using WordPress without disruption.

I’d done exactly what you are speaking of several times, and it works quite well if you do it this way.

Assuming that client.com is your client’s current web site. Install WordPress in client.com/wp

Get it working the way you like it there, then follow these instructions: http://codex.wordpress.org/Giving_WordPress_Its_Own_Directory#Using_a_pre-existing_subdirectory_install

The only difference is that you have an index.htm(l) already for the “old” non-WordPress site in the root of client.com. Switching from old to new is simply a rename of index.htm, to “reveal” index.php (WordPress). You can instantly switch back by renaming the old index.htm back to index.htm

This works equally well if you want to dramatically change a WordPress site. If you have /wp/ as your old site, you can create a new /wp2014/ site, then switch back and forth by simply changing the index.php file in the root.

No. That is handled with the change in “Site address” Setting, Permalinks save and the last paragraph “If you already have content in your site,…”.

Of course, during testing, you’ll see “http://www.client.com/wordpress/page/” style URLs.