When we here about Kauravas, we hear a lot of Duryodhana, Dushyasana, and Karna story itself. There were 100 Kauravas, so where were other 98 Kauravas excluding Karna, and what were their roles? What was their age differences?
What was the age differences between Kauravas brothers?
Gandhari gave birth to a "ball of flesh" after 2 years pregnancy. That flesh was divided into 101 parts and put into pots for 2 years. After thta Duryodhana was born and within 1 month, other brothers were also born:
The day that the haughty Duryodhana was born was also the birth-day of Bhima of mighty arms and great prowess.
Then, O king, within a month, were born a full hundred sons unto Dhritarashtra and a daughter also in excess of this hundred. ~ Mahabharata: Adi Parva: Chapter 115
Regarding your other questions, we find refrences for others as well but not that much. For example, we can find reference for Vikarana during Draupadi cheer-harana, war etc.