It seems clear that Bell would marry Margaret if she would take him. In conversation with his old friend Hale he says:
“Now, Hale; you know that girl of yours [Margaret] has got pretty nearly all my heart. I told you that before. Of course, as your daughter, as my god-daughter, I took great interest in her before I saw her the last time. But this visit that I paid to you at Milton made me her slave. I went, a willing old victim, following the car of the conqueror. For, indeed, she looks as grand and serene as one who has struggled, and may be struggling, and yet has the victory secure in sight. Yes, in spite of all her present anxieties, that was the look on her face. And so, all I have is at her service, if she needs it; and will be hers, whether she will or no, when I die. Moreover, I myself, will be her preux chevalier, sixty and gouty though I be.”
Elizabeth Gaskell (1854). North and South, chapter XLI.
This speech is rather facetious in its choice of images: it was in Roman triumphs that a train of captured slaves “followed the car [chariot] of the conqueror”, and a “preux chevalier” is a gallant knight in a medieval romance. But facetiousness can be a cover for genuine emotion.
Whether he was actually thinking of proposing is another matter. Bell clearly appreciates the poor prospects of his position: a “preux chevalier” in a romance admires his lady from afar; and he declares his willingness to stand aside for Henry Lennox, and even to favour the union with his money.
When Bell meets Thornton in the train on his way to Milton, he is more cautious about revealing his feelings: after all, they are mere acquaintances, not life-long friends. Instead Bell says that he would like to look after Margaret as a daughter:
“I would take a live dragon into my house to live, if by hiring such a chaperon, and setting up an establishment of my own, I could make my old age happy with having Margaret for a daughter.”
The point here is that Bell, a bachelor, cannot take an unrelated woman like Margaret into his household without causing a social scandal. He must marry first so that the situation is respectable. Note again the use of facetious imagery: by “live dragon” he means his prospective wife in this scenario.
“And there’s her Aunt, Mrs. Shaw. There might be a way open, perhaps, by my offering to marry that worthy lady! but that would be quite a pis aller.”
Marrying Mrs Shaw is the most plausible way for Bell to be able to make his household suitable for Margaret. But a “pis aller” is a last resort. What then is the first resort? Bell does not say, and Thorton is too caught up in his own thoughts about Margaret to pay attention to the clues that Bell is dropping:
Neither Mr. Bell nor Mr. Thornton was in a laughing humour; so the oddity of any of the speeches which the former made was unnoticed by them.
The question asks, “Would it have made a difference to Mr. Bell if John had revealed more of his love for Margaret?” I am not sure that it would have made any difference. Bell clearly suspects that Thornton admires Margaret:
“Do you know,” said Mr. Bell, wheeling round, and shutting one eye, the better to bring the forces of the other to bear with keen scrutiny on Mr. Thornton’s face, “that I once fancied you had a little tenderness for Margaret?”
And a sensitive man like Bell can surely read between the lines of Thornton’s reply:
“Mr. Bell,” said he, “before you speak so, you should remember that all men are not as free to express what they feel as you are.”