How to and how much to clean the HEPA filter?
There are washable filters and one that can not be washed, trust the manual about that.

Don't worry over or fight the small spaces that did not get clean. You will annoy yourself, and be limited in how much you can clean them if you try.

View them as a very small reduction in the filter area, be proud that you are one of the few taking proper care of your filter, and enjoy your clean air.