Curses are supposed to work even in Kali Yuga and currently we are going through Kali.
The following verse from Parashara Smriti (verse 27, Chapter 1) is relevant here:
Krite Tatkaalikah saapah Tretaayaam Dasamirdinaih | Dwapare
Chaikamasena Kallou Sambatsarenatu ||
A curse in Krita takes effect immediately,in Treta it does so in 10
days time,in Dwapara ...
Yes, Yakshas did penance and got boons from supreme Gods.
One such instance is given in Valmiki Ramayana: Bala Kanda: Sarga 25:
Once there was a formidable yaksha named Suketu, but he was childless, and he who had devout conduct performed a great ascesis.
Gladdening at his ascesis, Rama, then the Forefather Brahma gave him a gemlike daughter who is renowned ...
Yes. Kubera, lord of the Yakshas performed penance and received boons from Brahma. See here
Afterwards Kubera performed penance for ten thousand years in water with head submerged, to please Brahmā. Yet, Brahmā did not appear. Then he performed penance standing on one foot in the centre of Pañcāgni. Brahmā appeared and asked him to choose any boon. Kubera ...
Puranas do exist at those times. There are instances where Pandavas quotes from Puranas to prove that their arguments are dharmic.
One such event happens during the discussion regarding Draupadi's marriage.
During the discussion, Drupada, Dhrishtadyumna says that woman marrying more than one man is a sin. At that time Yudhishtira provides information ...