User Manual Himox H06 Air Purifier with H13 True HEPA

Yes. There is an instruction booklet that tells you exactly what to do. Just make sure you're connecting your phone to a 2.5 ghz wifi connection.

It is not good to rinse out the filter, it will destroy the structure of the HEPA layer. Then filtration will become bad.
But it is okay to clean the hair and pet fur on the pre-filter by dust vacuum or little brush. Cleaning the pre-filter regularly.

No, there is almost no noise in sleep mode and first gear, the third gear is a bit loud, like a fan, but its purification effect is great, I usually turn it on when I comb my dog or afer cooking.

They have restocked on the filters before. I am assuming some of the replacement delays are due to COVID shipping issues.
But that being said, you never know if they will get more if the manufacture will stop making the model you have to force obsolescence of your current model