Speaking of the origin of Henry Dashwood's £7000, there are plenty of possibilities swirling around.

He may have had some savings before his first marriage. As you say, he may have saved some of the interest on his first wife's funds. He got something from selling the Stanhill furniture. I'm guessing that Stanhill itself was either leased, or so small that the proceeds from its sale only made up part of the £7000. He may have saved something during his 11 years at Norland.

We can rule out Mrs Dashwood as a contributor, though, because she had nothing (ch1) and never saved in her life (ch6).

I'm not hugely impressed with Henry Dashwood's thrift, as it appears that he kept at least two of his own carriage-horses: The horses which were left her by her husband, had been sold soon after his death, and an opportunity now offering of disposing of her carriage, she agreed to sell that likewise at the earnest advice of her eldest daughter (ch5). Then again, perhaps his uncle paid their taxes and upkeep, thereby impoverishing little Harry.