Is a straight grammar error: the participe passé is not possible here, a conjugated verb is required. Well – this is a grammatically correct indicative present tense for the verb “ajouter“, first person singular :-/ It feels quite uncommon to me and the ending sound verb for this verb in particular made me think about a passé simple rather than a present tense. Also, the similarity to the participe passé made me think at first that this was a mistake and that a passé simple (which sounds the same) was intended here …

Feels correct to me, and appropriate for that situation. One of my favorites, feels natural.

Is probably grammatically correct, but the allitération in “qu” should be avoided. Gide would most probably not write like that. It sounds too clunky, and this “qu’est-ce que” feels like a lower language register. So it feels half-way between “elevated” and “familiar”, maybe more on the familiar side.

I am not sure whether 4 is correct or not. I think it is incorrect, and that we would use puisse instead of peut, even in regular language. I could be wrong.

Is probably ok, but the future tense clearly makes this about the future. That future might also be grammatically wrong. Not sure.

Sounds acceptable and ordinary indeed.

Probably acceptable and ordinary, except that now this talks about something in the future. It sounds a bit suspicious to me, just like 5. I would prefer “qui ennuierait“. The future tense might be wrong, not sure.

Yes, acceptable and ordinary, one of the first thing I would think about.

Not sure. There is something off with that one, or uncommon. It may be correct though.