First, let me say that most of these issues are when I'm working in a VR Stitch effect. But these are keyframe behavior issues.
Neither of your solutions does what I want. If I delete the last remaining keyframe, I lose the current value, it resets to the parameter's default. This can be hard to reset to the value I wanted. I consider this a bug.
If I set a keyframe to constant, With 1 keyframe, the behavior is essentially the same as having 1 normal keyframe. In both instances, the parameter remains in 'animated' status and will take keyframes as I adjust parameters.
This is a huge problem with an 8 camera stitch, suggested workflow includes using Match Color. Which will automatically set 24 keyframes. and every subsequent manual adjustment on a different frame will set 3 more keyframes - even with autokey OFF.
Match Color in Time should set keys. Match Color should honor the animated / non animated state of the 3 params it effects.
But in general, I want the ability to remove animation from a param and maintain my current value, not have it reset to a default.