I used Kdenlive in both modes for checking "strange" behaviours. This is what I found for the moment.

In the so called "Strict" mode (why not to call it "classic" mode?):

1. you still can put a video only clip on an audio track. If you separate on the timeline the video from the audio and then you move the video part on the audio tracks, Kdenlive accepts that you let drop the video part in the audio tracks.
2. audio speaker toggle on the video tracks are unuseful and create confusion.

In the so called "Mixed" mode:

1. you can select "video only" from the "Clip" menu though the clip is on an audio track on the timeline. The result is that you have a video clip on an audio track which is not played back. Actually, it's as you disabled it.
2. once you select "split audio" from the "Clip" menu, you can't reverse the action even if the clip is still grouped. There is a way to do it by ungrouping the clip and by selecting, always from the "Clip" menu, "Audio and Video". By this way, both the audio clip and the video clip can return to a video with embedded audio inside. The issue is that if you put some filters on the audio track or on the video track the new embedded derived clips loose part of the effects: if the new video/audio clip is generated by the video part you loose the audio effects, if the new audio/video clip is generated by the audio part you loose the video effect. Of course, as a workaround, you can copy and paste the filter from the previous clip before to delete it from the timeline but, for me, this is not too logic or intuitive.
3. As FCPX is the only proprietary software (I can't call it any more professional because professionals after the changes which make it more similar to iMovie have quitted it for AVID or Premiere) which uses also audio embedded in the video. I propose you to watch this clip about its new audio workflow: https://www.youtube.com/watch?v=QskPCj-Yj4s I'm trying to look for best practises in all major software so I guess that this can inspire us on for the fine tuning of the "mixed" mode. It's very interesting the window which FCPX uses for managing the audio stream inside the video. Of course, you can separate and embed audio and video all the times you want.

I have to remember that the "Configure tracks" window is still incomplete. You can't add tracks or reorder them.

I have 1 new notes on this topic:

Regarding to what I wrote on point 2 of the "mixed" mode section, the fact that even if you separate the audio from the video, both still contain inside the other missing part can be pretty useful. For eg, if I'm just using the audio or the video of a clip but during the job I need again the other missing part because I deleted from the timeline I can recover it immediately. For this reason, what I wrote at point 1 both of the "Strict" and "Mixed" mode sections is not correct because if you need to convert back the audio part of a clip in the video you have to do it on an audio track. Then the fact that you can separate infinite times audio tracks from the video is a very interesting feature.
For this reason, I guess we can delete the point 1 of the "Strict" mode section and points 1 and 2 of the "Mixed" mode section.

I found some interesting "issues" for copying and paste operations and other "strange" behaviours:

• Kdenlive doesn't copy clips in the target tracks but where the mouse pointer is also when the paste action is performed by the keyboard using the Ctrl+v keys. If you move the mouse pointer out of the timeline (on another window) Kdenlive uses as the reference the editline (cursor) and it inserts the audio part of the clip on the selected "Mixed" mode target track by ignoring the "Strict" mode green led targets though you are in "Strict" mode. The video part is placed on an upper track at the same distance the original clip was (for eg: you have the audio on Audio1 and the video on Video 2 and the target is set on Audio2 the video will be copied on Video1). If your target is a video track Kdenlive insert the audio there and the video part on an upper track. This happens also when you copy a block of several grouped clips.

I suggest that when you perform the paste action by the keyboard using the Ctrl+v command, Kdenlive has to always ignore the mouse pointer position and paste the clip at the editline position. If you are in "Strict" mode it has to respect the green led target. If there is no free space on the timeline Kdenlive has not to paste anything.
When you copy a group of several clips the targets for audio and video represents the starting point, the lower layer for audio and video clips.

• As Kdenlive pastes the clip where the mouse pointer is or in the "Mixed" mode selected track, you can copy a true audio clip (not the audio part of a video clip) on a video track. This will be no more possible if when you perform the command Crtl+v Kdenlive will put the clips on the "strict" mode green led tracks. Of course, in "Strict" mode, it'll be still possible to perform the action by the mouse using the right-click contextual menu. I hope that this option will be disabled in this modality.
• I found that you can convert a true audio clip by the menu "Clip > Video only" command into a blank video clip. I believe this has no sense.
• When the video part of a grouped video/audio "Strict" mode clip is dragged to an upper track, the audio part follows the video though it'll drop on a video track. I guess that once the audio part reaches the Audio 1 track you can't drag the clip further.

After the new fixes about the relation between audio and video track I found some improvements and some regressions:

As improvement, I really appreciated the fact that by using Ctrl+ mouse drag on audio or video track you can shrink only the audio or the video though they are still grouped. I tried to use the Ctrl+mouse for moving independently the audio or the video without ungroup them, but this operation crashes Kdenlive.

As regression, in Strict mode, I found that when you drag the video or the audio part of the clip, the other part follows vertically. For eg: You have the video on the video track 1 and the audio on the audio track 1. You drag the video on the video track 2 and the audio moves to the video track 1.
Or if you drag the audio on the audio track 2 the video moves on the audio track 1.
The correct behaviour is that if you move the video, the audio stays where it is inside on a audio track and when I move the audio the video stays where it's inside on the video track.
I guess that it's a no sense to have audio on video tracks or video on audio tracks. If we want to go in this direction maybe we have to remove the difference between audio and video tracks as it is on Blender but, I remind you, that Blender users complain for this situation because it makes very hard to work in a clean space.

I just found another "strange" behaviour: if you lock the audio track and you can't move the video part of the clip neither in the vertical way just for changing its track.
More: if the audio track is locked and I click on the video part of a grouped clip (or the video track is locked and you click on the audio part) I can't ungroup them because the menu is changed and you have no more this option. The only options we have are:
Insert space, remove space and paste but none of them actually works.