...would have been younger than his brother, or else Henry Dashwood would have inherited Norland from is father rather than his uncle.

I know that in Jane Austen's time, it was a usual practice to name a child after someone if it was hoped that the child might be that person's air.

We have Henry Dashwood, his son John Dashwood, and then John's son Harry (which would be a nickname for Henry). I always wonder if perhaps the old uncle's name might have been Henry, as well. Mr. Dashwood would have been named after his uncle because he would be the uncle's presumed heir. The fact that John and Fanny named their son Henry as well makes this all the more likely and was probably a shrewd move on their part. (John may have been named for someone on his mother's side of the family).