With regards to editing the ATOMS of an ALAC file, if you have the file in iTunes then iTunes will edit them. If you then share the same file with other programs they will be able to read those tags. I would recommend this approach if you are going to still use iTunes. However for external alternatives look at the following.
http://atomicparsley.sourceforge.net/
As you can see the name plays a pun on the ATOM aspect of MPEG4 files 🙂
http://www.softpointer.com/AudioShell.htm
Despite its appearance it does MPEG4 tags as well as MP3 (ID3) ones.
With regards to getting Windows Media Player to play ALAC files. You have found the right person 🙂 I was the person who came up with the very first solution to enable this and have ensured it is still possible despite Microsoft's best efforts 😠
The original solution was to install a DirectShow filter and a plugin to let WMP read MPEG4 tags. If like most you are now running Windows7 and WMP12 then you need to install the same DirectShow filter but this time it is best done as part of a codec pack which will let you disable the built-in WMP12 Media Foundation codec for (only) AAC. The Directshow filter can play both AAC and ALAC. If you don't disable the built-in Media Foundation codec then it takes presidence over the DirectShow filter.
See http://shark007.net/win7codecs.html
In theory you don't any more need a plugin to let WMP12 read the tags in AAC or ALAC files as WMP12 can itself now read MPEG4 tags. Unfortunately even though the file extension for ALAC is the same .m4a as AAC, and even though the tag format is the same, Microsoft wilfully chose in WMP12 to route ALAC files to the 'other' section rather than the music section.
Fortunately the developer of WMP Tag Plus was able to trick WMP and get round this problem.
See http://bmproductions.fixnum.org/wmptagplus/index.htm
If you install these two pieces of software you can get WMP12 to properly accept and play AAC and ALAC music files.
