Simple answer to your two questions
Is Nirvana a goal which every Buddhist must achieve? Yes. Ending the suffering is the ultimate goal of a true budhist
Is there any other things more important than attaining Nirvana? No
To answer this more, All budha's sutta and all teachings point to how to attain nibbana nothing else. In order to attain nibbana only thing you have to do is look into yourself and your sitha how thoughts come and go every second. Trick is not to attach to any of it because its impermanent. But if you get attached to these thoughts and feelings karma cycle begins in another words life begins at that moment.
Look at the Four stages of enlightenment and first step most important Sotapana.
The three fetters which the Sotāpanna eradicates are: (excerpted from Wikipedia for better explanation)
Self-view - The speculative view that a so-called self exists in the five aggregates (physical forms, feelings/sensations, perception, mental formations and consciousness) is eradicated because the Sotāpanna gains insight into the selfless nature of the aggregates.
Skeptical Doubt - Doubt about the Buddha, his teaching (Dhamma), and his community (Sangha) is eradicated because the Sotāpanna personally experiences the true nature of reality through insight, and this insight confirms the accuracy of the Buddha’s teaching.
Clinging to rites and rituals - Eradication of the view that one becomes pure simply through performing rituals (animal sacrifices, ablutions, chanting, etc.) or adhering to rigid moralism or relying on a god for non-causal delivery. Rites and rituals now function more to obscure, than to support the Right View of the Sotāpanna's now opened dharma eye.
The Sotāpanna realizes that deliverance can be won only through the practice of the Noble Eightfold Path.