Navkar (Namokar Mantra) and related fasts/rituals have nuanced, sect-specific rules. The exact practice of taking pacchkhaan (eating after/navkar) and whether one should brush teeth before Navkarshi varies between Digambar and Shwetambar communities, and even among local communities or teachers.

• Pacchkhaan timing: Some Jain communities observe a specific meal/food-related routine tied to daily worship, while others do not insist on a formal pacchkhaan before Navkarshi. The emphasis in all cases is to maintain ahiṃsā (non-violence) and cleanliness.
• Brushing teeth: Cleanliness is valued in Jain practice, but whether brushing teeth should precede Navkarshi or occur after a meal/ritual can differ. Many followers prefer brushing as part of personal cleanliness before starting any devotional routine, but this is not universal.

Because these details are tradition- and guru-dependent, the most reliable guidance is to ask your local Jain teacher or a trusted senior practitioner from your own tradition (Digambar or Shwetambar) for the precise rule relevant to your sangha.

If you’d like, I can help locate a page on JainKnowledge.com that discusses Navkar and related daily rites and point you to a source that matches your tradition.