Actually, everything is still being held firmly in place by pulling forces between the main board and heatsink, so I can’t imagine that anything will loosen over time. But, there’s no harm in using the nylon washers, if you prefer.
Hmm… not sure what to advise regarding cat hair. Canned air could be useful for occasional cleaning. But, I’d be careful about covering the Rock Pi w/o using cooling fans. Any restrictions to airflow could cause overheating, poor performance, and/or component damage. On the other hand, cooling fans can draw in more hair and deposit it inside the SBC. Perhaps a fan with a filter would work.
Good luck with that. I hope you find a suitable solution.
There are also some case designs (including my own) at the following topic that may be helpful for you.
Best regards