For example is it possible to be at the same time extremely high in conscientiousness and agreeableness?

There seems to be the conflict between the two, because a highly conscientious person who is dedicated to their work would, for example, easily refuse an invitation to a party, or really anything that would interfere with their work. Which isn't a very agreeable behavior.

Also, is it possible to be extremely high in conscientiousness and extremely low in neuroticism?

Again, there seems to be a conflict. Because a highly driven, ambitious, competitive person, would likely feel some tension about their results and would worry in case of failure. They would imagine all the potential problems in order to prevent them. And if someone's neuroticism is extremely low, could they be, perhaps too chill to care?