Willoughby hasn't said anything to anyone at this point that would contradict that, and although his behaviour to Marianne was very misleading (to her and her family), he also very carefully avoided mentioning the word love or marriage or engagement to her or to any of them.

The only thing that makes me wonder whether the Ellisons truly objected is the fact that it is Mrs. Ellison (as you said) whom Colonel Brandon overhears talking all about the wedding arrangements in the stationer's shop in Pall Mall. His account of her telling the story for anyone to hear and giving all the details to her friend does not sound to me like this was a wedding to which she objected or which she had tried to prevent:

"Two ladies were waiting for their carriage, and one of them was giving the other an account of the intended match, in a voice so little attempting concealment, that it was impossible for me not to hear all. The name of Willoughby, John Willoughby, frequently repeated, first caught my attention, and what followed was a positive assertion that everything was now finally settled respecting his marriage with Miss Grey -- it was no longer to be a secret -- it would take place even within a few weeks, with many particulars of preparations and other matters. One thing, especially, I remember, because it served to identify the man still more: -- as soon as the ceremony was over, they were to go to Combe Magna, his seat in Somersetshire. My astonishment! -- but it would be impossible to describe what I felt. The communicative lady, I learnt on inquiry, for I staid in the shop till they were gone, was a Mrs. Ellison...

Would Mrs. Ellison be in on all the pariticulars of the preparations and where they would go after the wedding, etc. if she and her husband had tried to prevent Miss Grey from marrying Willoughby? Would she be loudly talking about it in a shop?

We also have to remember that Mrs. Jennings' information is third-hand gossip at best: "I never heard any harm of her; indeed I hardly ever heard her mentioned; except that Mrs. Taylor did say this morning, that one day Miss Walker hinted to her, that she believed Mr. and Mrs. Ellison would not be sorry to have Miss Grey married, for she and Mrs. Ellison could never agree."

Look at the number of conjectures that are there before Mrs. Jennings arrives at her conclusion! It also sounds like, if any of that is true, that the Ellisons wanted her to get married.