Can anything (mostly anything) be learned with a lot of practice and dedication (career skill wise)?

Yes, if they practiced enough.

Yes, I think one can learn mostly anything with sufficient practice and dedication. There’s an old saying that all it takes to master a skill is 1000 hours of practice. I cannot vouch for the accuracy of it, but I once calculated the average number of hours each black belt in my karate school trained before reaching first degree (based on how many classes they went to each week during their training). It turned out to be around 1000.

If a person lacks certain basic attributes required for a skill, that will obviously preclude them from mastering that skill (or at least severely hamper their progress). But we must be careful about assuming what is required for mastery. A one-handed pianist may never play certain pieces, but may excel in unexpected ways nonetheless.

The person in that video is not himself limited to one hand, but the point remains that the piece can be performed by a person with only one hand. So can a few pieces that were not originally written for one hand.

If you had the financial resources, the time, and the perseverance to study and work at your courses, I’m sure you could become an astrophysicist, or a computer programmer, or an engineer. By a lot of your activity here, you’re a plenty smart dude :-)

Here is what I have become moderately competent in, with no instructor, over the past twenty-years.

Gardening
Composting
Birding
Driveway repair
Building a dry wall.
Home plumbing
Home roofing
Auto mechanics
Cooking without animal protein.
Tick removal
Cat care and maintenance
Meditation
Yoga
Understanding poetry
Computers and other parts of the new technology.

Malcolm Gladwell in the book Outliers posits that anyone can become an expert in anything if they practice it for 10,000 hours. The kicker is, of course, that you are not likely to spend 10,000 hours on something if you have no interest or innate ability in it.

Yes, they can, but most people would rather spend the time improving their skills on things they actually like – in other words things that come “easy” for them.

I have to disagree. I think most of us have certain natural limitations that would prevent us from reaching a level of expertise in some areas, just as much as we have natural abilities that give us an advantage in others.

For instance, my strengths lie on the side of the humanities and not the sciences. Some people have both, but I don’t. If I had nevertheless chosen to study, let’s say, medicine, I’m sure I could have mastered quantities of information and gained better than average technical skill. But I don’t believe that I could ever have taken to the art of it—the insight, the natural perception, and the imagination that lead to brilliance in diagnosis, exceptional resourcefulness in treatment, extraordinary problem-solving ability in the operating room, and so on. I think my aptitude might have taken me as far as rote and mechanical mastery and whatever skill comes from experience, but there would be nothing of the gifted or talented in my performance; I’d never be better than second-rate, tops.

So perhaps what it comes down to is whether you mean “skill” to take in everything it’s possible to do, right to the top of the profession, or, in contrast, that “skill” is defined as that which anyone can master with sufficient dedication (in which case the answer is a self-evident “yes”) but that true expertise transcends mere skill.