] Note that Mr. Gardiner lives within view of his "own warehouses," emphasis on the plural. He owns warehouses in the commercial heart of London. That alone establishes the fact that he's far better off than a mere shopkeeper or merchant.

Actually, it doesn't. "warehouse" was almost synonymous with "shop" and Gracechurch Street at the time was full of very posh shops

] The second clue is that the Gardiner family is able to spend each Christmas at Longbourne. Such a trip would most likely last at least two weeks,

One week, according to Ellen Moody's calendar. One could argue for longer, but the "twelve days of Christmas" was the usual hilday for all and sundry and it seems that most businesses took a break then.

Mon-Tues, Dec 23rd - 31st:
Gardiners remain a week at Longbourne, with Philips, Lucases, and the officers... not a day without its engagement; Jane goes to London with them Ch 25, p 180

] The third clue is that the Gardiners can afford their own carriage.(snip) Mr. Bennet, on the other hand, had to share his horses with the farm on his estate.

We don't know that either of these statements is actually true. First, Mr Gardiner could have rented the carriage and horses- carriage rental was big business in those days. Secondly, Mr Bennet "shares his horses withthe farm" because there was a heavy tax on horses owned simply for carriages, whilst farm horses were tax-free. Either way, Mr Bennet uses this as an excuse- we have no way of knowing whether he actually needed the horses on the farm, or whether he was just teasing.

] Furthermore, the Gardiners can plan to take a four-week pleasure trip, again with their own carriage and servants

A trip which is cut short.

In the end, the trip had to be shortened, but again what shopkeeper could even CONSIDER doing such a thing?

You might be surprised! There was no such thing as shop hours at this time, and many of the businesses shut down in the summer, when their clientele left for the country.