Yes and No. Per WOJ it's like any other Skill in the real world, like music or sports or whatever. Anyone can get better with dedicated work. Some are born with natural talent, but will always suck because they never develop it, while others are born with enough disadvantage (ie akin to being tone deaf) that they'll never be able to match the greats with hard work. But that's not to say that a very dedicated person couldnt reach the Council eventually. Their magic likely wont much resemble Harry's brute force approach or even Molly's natural grace, and would likely lean more toward the study and slow ritual approach.
To a point, yes. But there are limits. A person with tremendous skill and modest natural strength might make the Council after many years, if they practice and focus and work at it. But you have to have the minimum level of strength, or it's got going to happen.
Remember, Council-level magic isn't just the best, it's the best of the best. Playing at that level is going to require a certain minimum amount of available energy, no matter how skilled you are. Some Council-wizards have lower native strengths than others. Molly is jealous of Harry's raw strength, and even Anastasia admits that she can't come close to matching Harry for 'bench-pressing' magical energy. But pretty much any Council wizard is going to have to be born with both a good dose of strength and a minimum level of natural talent, or it won't happen no matter how hard they try.
It's like a 5'0" skinny guy who practices really hard, really refines his skill, until he's an awesome basketball player...for a 5'0" skinny guy. He still isn't likely to ever play for the Chicago Bulls no matter how hard he tries.
In this context, the Bulls are like the White Council. Who can be 6'9" and in good shape, but you'll never play for the Bulls unless you work out and practice hard and long even so. You can be 6'9" and work hard and get skilled, but fail to make the Bulls because you're so nearsighted. Or you could be 6'9" and have great vision and work hard but also just naturally clumsy. No Bulls contract.
To make the Council, you've got to have the basic strength, the skill to use it, the will to learn the skills and maintain them, the self-discipline to use the magic and not break the Laws, the will to hold your own with the other Wizards, your talents have to be general enough that you're not just a magical
idiot savant, the brains to have that kind of power and not blow yourself up, etc.
Remember back in the early books when Harry compared himself to a magical thug, and he and others contrasted his clumsy magic with mages like Elaine, or other Council members? Well, he was a magical thug back then,
by Council standards. But the fact that he was on the Council means he was more delicate, more skilled, more refined, even then, than 99% of all magic-users.
When you add all that up, what you get is that Council-potential almost has to be inborn in reality, whatever the theory is.
