What does "dharma" mean? And is dharma created by God, rishis, and the devatas, or is it eternal and always applicable to all of humanity, all the time?
Dharma is indeed created by Brahman, rishis, and devatas. This is known because dharma is based on Vedic vidhis (injunctions). Each vidhi was authored at a certain time in history. This means that before the vidhi was authored, the action enjoined or prohibited by the vidhi was not dharma.
There are also many instances recorded in the Vedas themselves as well as in smriti where dharma is created. Hence, dharma changes based on time, place, and audience.
The first incident is Ushanas' banning of liquor consumption among Brahmanas:
The learned Sukra, having been deceived while under the influence of wine, and remembering the total loss of consciousness that is one of the terrible consequences of drink, and beholding too before him the handsome Kacha whom he had, in a state of unconsciousness, drunk with his wine, then thought of effecting a reform in the manners of Brahmanas. The high-souled Usanas rising up from the ground in anger, then spoke as follows: "The wretched Brahmana who from this day, unable to resist the temptation, will drink wine shall be regarded as having lost his virtue, shall be reckoned to have committed the sin of slaying a Brahmana, shall be hated both in this and the other worlds. I set this limit to the conduct and dignity of Brahmanas everywhere.
It appears from this passage that liquor consumption of Brahmanas was not sinful at one point, and that Ushanas, aka Shukracharya, the Brahmana and Rishi Guru of the Asuras, made it sinful through his proclamation after finding out the negative effects of being drunk.
The second incident is Uddalaka's son, Shvetaketu, banning polyamorous relationships out of anger from seeing the negative effects it caused:
But I shall now tell thee about the practices of old indicated by illustrious Rishis, fully acquainted with every rule of morality. O thou of handsome face and sweet smiles, women formerly were not immured within houses and dependent on husbands and other relatives. They used to go about freely, enjoying themselves as best as they liked... The Rishi's son, Swetaketu, however, disapproved of the usage and established in the world the present practice as regards men and women... Accordingly, since the establishment of the present usage, it is sinful for women not to adhere to their husbands. Women transgressing the limits assigned by the Rishi became guilty of slaying the embryo. And, men, too, violating a chaste and loving wife who hath from her maidenhood observed the vow of purity, became guilty of the same sin.
And this rule was created by Shvetaketu out of anger from seeing the negative effects this practice caused in society:
... the present virtuous practice hath been established by that Swetaketu from anger. Hear thou the reason.
Once again, we see a powerful Rishi create the laws of Dharma on what is sinful and not sinful.
Another important point to note is that the original Kamasutra was actually written by this very same Shvetaketu, so it appears that he wrote the treatise in this highly sexual environment.
Shvetaketu is also involved in the conversation on the Panchagni Vidya (science of reproduction) in the Brihadaranyaka Upanishad:
A fire – that is what a woman is, Gautama. Her firewood is the vulva, her smoke is the pubic hair, her flame is the vagina, when one penetrates her, that is her embers, and her sparks are the climax. In that very fire the gods offer semen, and from that offering springs a man.
From all this it appears that Shvetaketu was an expert on sex and eroticism, so this section in the MB should be regarded as authentic.
Another incident is Indra's distribution of 1/4th of his sin of Brahmahatya to women. This distribution caused their menstruation cycles. From that point onward, the husband and wife have to observe some rules during menstruation. For example, it is a sin to eat the food cooked by a women during her menstruation. Also, when Indra gave his sin to women, the women in return asked for the boon that they can have sex with their husband whenever they want, even on forbidden days, and the husband must comply. So now, it's not sinful for wives to ask for sex anytime, and it's sinful for the husband to not comply with her request.
And Apastamba has forbidden Niyoga in kali yuga:
126.96.36.199 - That (niyoga) is at present forbidden on account of the weakness of men's senses.
One can find more examples from a thorough reading of shastras.