Barbara> Does it really seem to anyone, upon a close reading of Chapter 1, that Henry Dashwood ought to have realistically expected to inherit anything more for his wife and daughters than what they got?

Oh yes indeed.

When the narrator tells us that such attractions as are by no means unusual in children of two or three years old were enough to outweigh all the value of all the attention (ch1) of Mrs Dashwood's branch of the family, there's no mention of the grand old family's prosperity being 'on the scales'.

Even if Henry Dashwood thought it was a consideration for his uncle, the Dashwood dynasty's wealth has lately grown by two big dowries, so there's plenty of room for generosity.