Takes about 4 to 5 weeks minimum for viable seeds. Harvesting any sooner and you'll risk undercooked genetics. (seeds that aren't quite right genetically, or don't germinate)

Depends on the age of the plant and other insignificant stuff, but the yellowing is from the plant being in flower and using-up the nutrients, and all the other crap that fan leaves store. (leaves are a nutrient/chemical storehouse) Were it strictly from lack of light, we wouldn't get secondary buds.