The kernel of the question
I recently said:
Je ne suis pas tout à fait certain de ce que ces phrases soient naturelles.
Later, @Laure helpfully suggested this simplification:
Je ne suis pas tout à fait certain que ces phrases soient naturelles.
Sure enough, this Ngram shows certain de ce que absolutely flatlining compared to certain que. But This one shows that content de ce que is used, albeit less commonly than content que. I'm surprised to see that they're so different! What should I gather from this correction?
Does "être [adj.] que" alternate with "être [adj.] de ce que" stylistically, perhaps along the lines of classic vs. modern?
Is être [adj.] de ce que" regarded as incorrect?
Does it entirely depend on the adjective?
Which of the above explanations is correct (or is another one)? What else should I know about de ce que and à ce que in contexts like these?
Background / what I've learned so far
After having searched a little to see whether my question has already been answered, the closest I found was this question, which I'll build on in the belief that the à ce que, de ce que discussed there works like the one above.
To summarize what I've gathered: This particular construction ce que preceded by a preposition introduces a verbal clause, generally in the subjunctive. It completes certain expressions and can also explain the alternative forms of expressions like "de manière (à ce) que" and "rapport à ce que". (However, it's unrelated to ce que "COD + which".)
This ce que is mandatory in at least one construction that alternates with a nominal or infinitive complement:
→ Jusqu'à ce que l'année soit / sera terminée.
→ Jusqu'à la fin de l'année.
→ Rire jusqu'à en mourir n'est pas une blague.
At some point during my acquisition of French, based on the first of the above examples, I apparently generalized the rule along these lines:
Je suis content de ton succès.
Je suis content de ce que tu aies (as ?) eu du succès.
J'ai peur des monstres.
J'ai peur de ce que les monstres viennent.
Ils s'attendent au jour de son retour.
Ils s'attendent à ce qu'il revienne.
This brings me about up to where @Catomic was in the question I first cited.
Now, Catomic had found a sentence like one of the above and was wondering why it wasn't:
Ils s'attendent qu'il revienne.
In Laure's answer to Catomic, I learn:
There's absolutely no difference in meaning between [s'attendre à ce que] and [s'attendre que ...] the difference lies in what is considered a classical use [que] and Gide's modern use [à ce que]. In the TLF (III 3) we can read that both s'attendre que and s'attendre à ce que are correct but that purists advocate the former and the latter is more usual.
I also learn there that if there is a difference in usage, it's that à ce que variant can use l'indicatif du futur simple, as I did with sera above — at least according to Grévisse (Laure at least thinks that sounds odd).
Now @jlliagre, in his answer, draws a distinction between the behaviour of different verbs, e.g. se préparer and se souhaiter :
Je me prépare à ce qu'il y ait la guerre. Correct
Je me prépare qu'il y ait la guerre. Incorrect
Je me souhaite à ce qu'il y ait une bonne guerre. Incorrect
Je me souhaite qu'il y ait une bonne guerre. Correct
And his mention of pronominal and reciprocal verbs suggests that the patterns apply differently to different classes of verb.
So I can't necessarily generalize Laure's notes on s'attendre as classic vs. modern usage because I may run into simply incorrect uses depending on the verb.
That brings us up to my current question. I know that être [adj.] de is a different kind of phrase from [verbe] à, but I was hoping that the analogy would work on the common ground of "ce que + [groupe verbal]".
It would certainly be interesting if être certain que was the modern use and être certain de ce que was classic/outdated, given that the reverse was the case with s'attendre que vs. s'attendre à ce que.
Am I on the right track? I guess what I'm asking is, in practical terms, what can I use, and what should I use, in what contexts, when constructing this kind of sentence?