Hello Dan (and everybody who is struggling a bit with the same issue I had)
I think I discovered a very, very nice solution to my problem.
I am not sure if this is only on version R21 of C4D (I will check later on my R18 perpetual license later on)
But I realised after I removed the keyframes when creating a new motion clip, a new timeline appears on top of my main character animation, folded, with the name of the motion clip I just created (in this case, Walk Cycle)... when opening, I can see the entire embbed animation from the motion clip in there (the same as accessing Dope Sheet from the motion clip timeline) so you can do your fine adjustments there.
So far, nothing new... but of course, you can adjust the settings over there, so there is not really a need to convert the motion clip back to keyframes, since it is already easily accessible from there.
But.. when you decide to add a new keyframe element (for instance, rotation on X axis, but you forgot to make a keyframe for that when creating the motion clip. you can leave your motion clip on as a reference for the new keyframe, or not, and animate on your main character on its own timeline (this will be the only animated bit since everything else was taken inside the motion clip):
And after you are done, you can simply drag this new animated element inside the motion clip timeline!
The new motion clip will automatically contain this new animation setting, in a very non-destructive manner!!
of course, if you animated LEFT_FEET rotation on X axis, you drag it inside the "WalkCycle" motion clip LEFT_FEET rotation tab as well.. but if you want, you can drag it anywhere you want, pelvis, etc.. and it will still work!
This is just amazing!! Next level stuff. This will definitely help me a lot with my character animation productions from now on. Being able to keep snippets of my animations, adding things here and there and keeping it overwall, being able to match animations,etc.. damn.. I have no idea the level of programming magic that goes into this! But I am so happy this exists on C4D
Thanks again Dan!