WHen you get down to the album level and can't drill down any more double-clicking to play it is the logical next step. If you want to see what files are in there you click it.
Why can't I drill down to the file level? I'm not sure I follow you. Why should the [Album] step always be the bottom level? When I want to see the files inside the album, I double click the album's tile and it shows the files (assuming your view scheme is [Artist]/[Album]).
But you'd be browsing your media thru the Video branch, not the Audio branch.
The audio, video, and images branches all work the same way, so the same rules apply. When browsing video and my view scheme is set up with a [Genre]/[Artist]/[Album] structure, the old way was unusable because double clicking on the [Album] tile would play the entire season of shows. Again, if you think of the tiles as folders (which they act like much more than files) the double click behavior should open the folder and display the files. If you want to play all of the items in the folder, you should have to select the folder and click "Play". I'm not sure what the problem is...
Also MC has the settings "Double-click: Replace Playing Now (all)" option which is set but it's being ignored. Either remove the option or honour my user preference.
This is not being ignored. If you double-click on a
file, it replaces all of the previous items in the playing now list with all of the files in the list currently showing when you performed the double-click action. This is just like it always was. The difference is that double clicking on an Album tile does not add those files to Playing Now, it instead opens the album up and displays those files.
If it doesn't and it shows some other category, you need to adjust the View Scheme you are using so that [Album] is at the bottom of the "chain".