I think Mrs Elton is being cheap - it is a lot easier to buy a couple of packs of cards, and extra candles, and have an evening party, than to give dinners like the ones that are being put on for them. (What an exactly so for Mr Elton's showy but cheap improvements to the vicarage. Did ever a couple deserve each other more?)
Wax candles were more expensive than tallow candles, and they burn cleaner, brighter, longer. Spermaceti (from Whales) made very expensive candles. Paraffin (the wax in modern household candles) was first distilled by Laurent in 1830.
I don't think Mrs Elton would be game to use slaves for waiters, even if she could get them - Slavery was unfashionable, and Mrs Elton does care about fashion.
There had been some important judgments concerning slaves in Scotland, - James Montgomery, James Somerset, Joseph Knight - that made it a bit difficult to keep slaves in the UK (as opposed to the colonies). JulieW has studied this interesting aspect, so I'll leave it to her.
Anyhow, Slavery was unfashionable, and Mrs Elton does care about fashion.