Wolcott,
Some info that might help you:
1. Sometimes when you make a change to a tag value, MC doesn't react fully to the change right away. It isn't really a mistake but rather a choice about how the user interface should work. For example, when you edited the album tag removing the CD number, MC no longer had the old album value in its database. However, if it immediately removed the album name and cover art from the list in the upper area and since no album was now selected, displayed all files from all albums, you might be confused about what happened. Sometimes MC continues to show the old album tag vallue and cover art and list the contents of that album. You see the same list of files that you saw before the album tag change.
MC may not fully update what it displays to reflect the tag value change until you leave the view and come back.
2. The Play button is overloaded. That is, it has more than one function and at any one time, it may serve one purpose and not the other.
If you click on the play button while MC is not playing anything, it starts playing the first track among those you have selected. (If you haven't selected any files, it treats them as those all the files listed were selected.) The other files selected are queued up to be played after the first tract is finished.
Once MC starts playing a file, the play button works as a pause/stop button. (Like a tape recorder.)
However, you can select some files, right click with the mouse over one of those files, and choose "Play" or "Add to playing now". Chosing "Play" causes MC to immediately replace the track currently playing by the first file in the group you selected and replace any tracks in the queue by the rest of the saelected tracks. Choosing "Add to playing now" causes the selected tracks to be queued for playback after everything already in the queue.
As MrC said, Experiment.
Bill