If the signal is strong enough, it doesn't matter what freqency it's on, it can overload the front-end of the Rx. At that point, all the fancy hopping schemes and filters are moot. If that's the case - find a new place to fly!
ER9X doesn't need cert. FrSky has the certificates on their website.
(ER9X and FrSky on a 16ch setup is in my Flanker