IMHO, this has gotten too complicated, unless you are flying over dirt or fine abrasive debris, you do not need an air filter. The blowback "reversion" issue is very common and is typically cured by fitting an intake velocity stack. They are commercially available from numerous sources in metal or plastic or can easily be made. This my thought on the issue. Good luck.
Here is a link to one, but a search for intake velocity stack should bring up a lot of options
http://www.ebay.com/itm/Zenoah-Stock...AAAOSwNNxWFRdb