Your sentence has the word 「容易」 which has both -な形容詞「容易な(yooi-na)」 and the -い version 「容易い(tayasu-i)」. 「容易さ」 in your sentence can be read either "yooi-sa" or "tayasu-sa". The latter is generally used and the former is rather unusual. The problem is, now all the difference could be between 容易い(tayasu-i) and 容易な(yooi-na), not -さsuffix and -性suffix. Still, I will try to thnk about what's the best choice for your sentence.
First, I don't think 「容易い(tayasu-i)」 (and naturally 「容易さ(tayasu-sa)」) can be used in that context. So, I would rather pick 「容易性」 there. A possible reason is that 「容易い」 has a similar meaning as 「簡単な」. These adjectives can be used only when describing an action/what you do. An incident just happens by itself and is by no means an action. (This reasoning is just my opinion and thus open for objections)
As for the 「頻繁」 part, I would say 「頻繁性」 is not really a valid word. However, this word does not sound as awkward as other obvious mistakes. So actually, many people may think it acceptable and only few people would complain. However, it's still worth avoiding the use of 「頻繁性」.「頻繁さ」 is okay to use, but -さsuffix makes things sound a little casual compared to -性suffix, not to mention you already have 「容易性」 for a preceding word. So I recommend you to choose a word with a -性suffix that is a synonym of 「頻繁さ」.
All things considered, I would go with