unable to manually manage music on my iPhone 4s

With iTunes Match, the whole "sync only checked songs and videos" option doesn't apply. With iTunes Match, you are not doing any syncing. Only streaming/downloading content to your phone.

With iTunse Match, all of the songs you see in your music app on your iPhone are in the cloud, not on your phone (unless you click the button to download them, or I think play the song). That's the purpose of it, to keep your phone's space open and just stream your content. Now, the way iTunes Match is supposed to work is that you can stream all the content, that's the way it's set up. When you first enable iTunes Match on your phone, it wipes all the music currently stored. From there, you stream songs unless you choose (by hitting the download button next to a song/album) to download them. On iOS 5, you were able to delete songs from your iPhone that you had downloaded with iTunes Match.

Now, where I see your problem lies is with a fault of iOS 6. As of now (and until 6.1 comes along to fix it), you can stream the songs as normal, or download them. However, once they are downloaded (on iOS 6), you cannot delete them. This will be fixed with iOS 6.1 (as per the beta).

 

You can delete them. Just not individually. You can go into settings and remove all the music.