When the lame carriage-horse is mentioned, "Mrs Elton was growing impatient to name the day, and settle with Mr Weston as to pigeon-pies and cold lamb"(42), and why would she, if she knew her carriage horse was not yet up to the task?

"It might be weeks, it might be only a few days, before the horse were useable"(42) and if Mr Weston were pressed by a lady determined to go to Box Hill sooner than later, what would his sanguine temper suggest as the most likely time-frame? And when the horse failed to mend as fast as he had thought, what could be more natural than for Mr Weston to discover that they could do very well without the carriage and arrange things accordingly?

It also seems more likely to me that Mrs Elton would find the lameness of a carriage horse merely the "most vexatious"(42) delay of a most frivolous errand that could be undertaken just as well a month later, because it was not affecting her ability to get around or increasing the expense of her stables.
When she uses the plural "These delays and disappointments are quite odious"(42) for the singular issue of one lame horse, to magnify the inconvenience to herself, it seems more likely that she would be talking about something she can clearly slot home to the bad management of other people and their inferior stablehands.