Mr Collins, the hero of breath control! I wonder how he manages to speak such long sentences:
She is unfortunately of a sickly constitution, which has prevented her making that progress in many accomplishments, which she could not otherwise have failed of, as I am informed by the lady who superintended her education, and who still resides with them. (chpt 14)
Allow me, by the way, to observe, my fair cousin, that I do not reckon the notice and kindness of Lady Catherine de Bourgh as among the least of the advantages in my power to offer.
But the fact is, that being, as I am, to inherit this estate after the death of your honoured father (who, however, may live many years longer), I could not satisfy myself without resolving to chuse a wife from among his daughters, that the loss to them might be as little as possible, when the melancholy event takes place -- which, however, as I have already said, may not be for several years. (chpt 19)
His excitement shows up by contrast when he encounters Darcy at the Netherfield Ball and we suddenly have a mix of short sentences:
How wonderfully these sort of things occur!
"Indeed I am. (chpt 18)
He does talk like an essayist. Points are laid out, parenthesis included. I wonder if this was drilled into him at university?:
"My reasons for marrying are, first, that I think it a right thing for every clergyman in easy circumstances (like myself) to set the example of matrimony in his parish; secondly, that I am convinced it will add very greatly to my happiness; and thirdly -- which perhaps I ought to have mentioned earlier, that it is the particular advice and recommendation of the very noble lady whom I have the honour of calling patroness. (chpt 19) [emphasis mine]
JA brings out his humility, combined with his self-conceit, in his language:
my humble abode (chpt 14)
"I am by no means of opinion, I assure you (chpt 17)
permit me to say
give me leave to observe
You must, therefore, allow me to
Pardon me for neglecting to profit by your advice (chpt 18)
allow me to assure you
Allow me, by the way, to observe
I flatter myself it will not sink me in your esteem.
I am not now to learn
You must give me leave to flatter myself (chpt 19)
In all this he claims to be humble, but he doesn't give anyone pause to change his mind.
For Mr Collins, it's all about him. He brings up the things he is proud of all the time. Every conversation comes back to him. Mrs Bennet asks him about Lady Catherine, and he takes the conversation to Miss Anne driving by his home and then his ability to flatter young ladies.
When Mrs Phillips asks him to play cards, here's another opportunity for Mr Collins to talk about himself:
...I shall be glad to improve myself, for in my situation of life -- (chpt 16)
Mr Darcy is of course to be pleased by Mr Collins introducing himself.
I am most thankful that the discovery is made in time for me to pay my respects to him, which I am now going to do, and trust he will excuse my not having done it before. (chpt 18)
His talk in chapter 19 is at first of how he will benefit from marrying Elizabeth (although he does offer Elizabeth the attractions of Lady Catherine de Bourgh).
However, Mr Collins does require me to drop my hypothesis about sophistication of language use being a sign of people's intelligence in JA's novels:
humility of behaviour
dictates of my conscience
habitual study (chpt 18)
sanctioned by the express authority (chpt 19)