It's a very interesting question, and I can't speak to the science but I can speak to my personal experience.

Going back to childhood I always remember the adults insistence that I decide which is my handedness yet knowing intuitively that I could favor either side, and they each had an advantage.

I played left wing in hockey, only because there was never enough left-handed players, so I just pretended I was left-handed.

When I would play little league baseball, coaches would shout at me saying hey, don't you bat the other way?! To me I just naturally, almost randomly picked a side according to how I felt about the pitcher.

When I played snooker semi-professionally, I shot right handed, but not because my right arm had more finesse, but because my left arm was better at providing rock solid stability with fine control, and because my right eye is slightly stronger. In my life, I played perhaps 50 games of billiards left-handed, out of perhaps 20,000 games total. And I can pick up and play left-handed with ease... You would think I'd been shooting that way my whole life.

I use my right hand to write, but when I skateboard, I skate "goofy foot". When I destroyed my shoulder and it was a piece of meat hanging off my body for 6 months, I picked up a pen in my left hand and within 3 days I was writing at the same grace I could in grade 6! Within a month I was actually writing better than right-handed. It was still chicken scratch so I'm not sure what that's worth lol

I know that I am right-handed by choice because there's a difference in the knuckle/tendon of my left thumb. It makes it impossible to move from certain positions on the "circle" to others without first moving to a transitional spot. And I have more dexterity with my right for that reason alone.

I always wanted to play on P2 of the Street Fighter II cabinet.

I think those people would be labelled as clumsy or lacking motor skills. The brain is pretty good though so with experience it can almost always figure stuff out.

Jessica Cox is a certified scuba diver, a light sport pilot, and I think it is safe to say she is without handedness.

There's a word for it.

Ambisinister

https://www.dictionary.com/e/word-of-the-day/ambisinister-2021-08-13/

I mean, depending on the task, I have felt this. There are sometimes things I can't figure out which hand to use because both feel wrong. Not often. Guitar feels like that for me.

I also read that as we get older, we become less "handed" and it's not because we become ambidextrous just less dextrous overall, the dominant hand loses dexterity.

Are you asking whether there are clumsy people, and people who feel clumsy? Yes, yes there are.

I am right-handed, and I tought myself to use my mouse with the left hand when working on my laptop.

The reason for that is that I have a couch, where the ottomane (the "long" part where you can rest your legs on) is attached to the right side (referenced to my seating position), meaning that, when sitting on this side of the couch, the arm rest of the ottomane is to my right side which doesn't leave enough room to operate the mouse without obstruction.

The side left to me (where the rest of the couch is), is unobstructed and leaves enough room to place and operate the mouse there.

At first, it was hard to navigate with the non-domiant hand, and I used it for navigating within the web browser. The majority of mouse navigation in a browser is scrolling anyway.

After a just a few of weeks I noticed that handling the mouse with the left hand became more and more precisely. Now I use my left hand exclusively with the mouse. I even noticed that when doing stuff in Blender or Affinity for example, keyboard shortcuts are more accessible to me with the right hand when working with a laptop.

When at work however, I use the mouse with my dominant (right) hand, as the desk layout allows me to do that.

A version of what you are saying is called cross dominance. Where a person is "handed" but users different hands for different things. For example, I write right handed but play sports and shoot left handed. I use left handed scissors but right handed hammer, screwdriver. All of the things feel awkward with the wrong hand but that hand changes with the task.

The closest I can think is someone who is ambidextrous but also dyspraxic.

Me playing the piano when I've never played the piano