How can we correctly think of the history of 成？ It is used as a result-complement as well as a meaningful bound form （成人，成长，成功，etc.). Which usage is original and gave rise to the other?
This is a very historical character originating from as early as 1600 BC. The initial meaning of 成 has always been in arguments, it's just too old.
So, I'm afraid that neither of the two semantics is "original" for such an old character.
In the oracle bone scripts, the character looks like a giant axe hitting a wooden stick or a stump. It is inferred, by some historians, that this oracle character is used to symbolize the fulfillment of an oath, because ancient people might typically sword against a stump or its like to leave a mark, whenever they make a promise, as if the scar on wood would witness their words. So maybe we've found the original meaning of 成: fulfilling -- that's a verb, not a result complement or a bound form.
BTW that "axe-like" part also evolves into a Chinese character 戌, which means:
- a kind of axe-like weapon with wide blade, used to butcher criminals;
- the "eleventh" in twelve, according to the Dizhi;
- dog in the twelve Chinese zodiac signs;
- the earth element in the Wuxing theories;
- honesty and sincerity
成 is typically a verb in ancient Chinese, it means "accomplish", "succeed". It can also mean "allow something to succeed" or "let something succeed", which is generally considered as a causative usage of 成. Note that such causative usages are so common in ancient Chinese that it's not rare that they finally integrate into, or become part of, the character's original meaning, which explains why 成 can also mean "approval".
成 as a verb is also very common in modern Chinese oral language. It more or less means "accomplish", "approve" or "turn into":
- 成！: (as a slang) okay; I agree to do that
- 成了！: (as a slang, basically the past tense of 成) there we go
- 不成！: (as a slang, = 不行) nope
- 难道 [any declarative sentence] 不成？: (fixed usage to construct a rhetorical question with emphasizing tone, can be quite emotional) it can't be true that ...
Other usages you may want to know:
- 成事: (verb) do anything well (no specific job, just to imply a general tendency to succeed); eg. 1. 这个人能成事 (Seems that this man can do anything well, i.e. You can count on this man); 2. 成事在人 (It is people that make things succeed; i.e. We emphasize the importance of human beings, and their hard work, rather than luck or gods' help)
- 成仁: (verb) sacrifice one's life, for people's welfare, or for a righteous cause, or for honor; eg. 1. 杀身成仁 (die for a righteous cause); 2. 不成功便成仁 (Succeed, or die with honor)
- 成佛: (verb, in Buddhism) touch the spiritual enlightenment; become divine