I will take a stab at answering your questions, in turn.
import all my mp3 from the Samba share (that mt-daapd is accessing) into the Library in iTunes
Yes and no. How is that for a definitive answer? 😉
It is not necessary to import them, per se. Your best bet is to select the directory containing your music on the Samba share as the "iTunes Music folder location" in the iTunes preferences. Once you have done that, locate that directory and drag it onto the "Library" in your source pane. That will catalogue all of your existing tracks within iTunes and allow you to manage them as needed.
Step one will allow for creation and management of playlists, yes.
find the .xml file and copy it to the mt-daapd mp3 root
You can do this, but the playlists will quickly fall out of sync when any subsequent alterations are made. My suggestion would be to mirror the expected iTunes directory hierarchy on the Samba share (i.e. "iTunes" —> "iTunes Music" —> <music files>. Then, if you designate the "iTunes" directory as your MP3 source in the mt-daapd config file, the daemon will parse the XMl playlist file and publish it with the share.
restart mt-daapd (necessary or will a periodic scan see it?)
Originally, I thought that playlist rescans required quitting the daemon and restarting it. Recently, though, someone corrected me on that point by stating a rescan would grab playlist changes as well as the addition of new files. YMMV.
If so will this actually copy the mp3 files to a new location (I want to keep them in one central place)?
No, not if you follow the directions above with respect to pointing iTunes to the Samba share as the location of the iTunes Music folder and Library. This same configuration can be used with every iTunes client in the house, if you like, for management of the library, playlists etc. (see below for the caveat)
There is no automated way of syncing the iTunes library when new mp3s are added to the Linux box?
If they are ripped and added via iTunes, then, yes, this is possible. If you make use of another application for ripping and adding or if the files are not added through iTunes, then no, there is, TMK, no way to tell iTunes of their presence. mt-daapd will pick them up on a subsequent scan, but iTunes will remain ignorant — unless and until you manually locate and dump them on the "Library" source.
We will have copies of iTunes (or whatever) on on my PC and my wife’s, so we can both create playlists. Is that possible? I guess two .xml files.
Yes, this is possible and no, there will not necessarily be duplicate copies of the XML files. Following the procedure outlined above will allow all iTunes clients to make use of the same management files. The caveat being that only one of you can make use of iTunes to do so at any given time. iTunes puts a lock on the "iTunes Library" file. Attempts to access it from a client on another machine will return an error to this effect. This is the only drawback to this scenario that I have encountered in over three years of similar usage. To me, it is a minor and acceptable limitation. Of course, YMMV.