] So, my guess is that not only did he start out with more money, he has been very succesful in his business (whatever it is). His children are likely to be raised as ladies and gentlemen

There is NO evidence to support this. The Gardiners are middle class. Their children are likely to be raised as middle class boys and girls are raised. Why make them out to have more than is likely.

Taking the view that they are closer to gentry removes the vast difference between them and Darcy.

Elizabeth at one point believed that her Aunt and Uncle would have been lost to her. They are not people "of fashion".

Remember that Mr. Gardiner didn't actually do anything. Darcy did. There is nothing in the text to suggest that Mr. Gardiner's finances wouldn't have been very poor after giving Wickham 10 grand. The assumption was made because it was reasonable given the evidence Lizzy and Mr. Bennet had. But they still worried about paying him back.