Is obtaining a university degree in this manner unlawful (haram), and consequently, is any job acquired through it invalid?

Cheating in exams and copying assignments, if the requirement was for them to be solved individually, is a forbidden act. One must repent to Allah for that. As for working with a university degree obtained through cheating, there is no harm in it if the person is qualified for their job and performs it with sincerity and proficiency, and had repented from the cheating.