Yes, Mrs. Jennings was John's mother in law. The actor played Siegfried Farnon in the old All Creatures Great and Small series from the 1970s. He was jolly in that one, too. I like him a lot.
I think his wife passed away- but he got on so well with his mother-in-law, that she stayed on. Lol. THERE's a twist. Hugh Laurie played Mrs. Jennings other son-in-law married to the chatterbox Charlotte. I love Hugh Laurie. His facial expressions are amazing. He'll always be the last of the Woosters to me. Those books are hilarious, if you have not read them. Definitely "light" reading. The P.G. Wodehouse collection.
Yes, they were very good the their cousins, even if they were gossipy. They were quite generous. It's been so long since reading, but I don't know that I like it better than P&P. I think P&P was wittier.
The film about Jane Austen is called 'Miss Austen Regrets'. I don't know how accurate it is, BUT I do know that Cassandra burned many of Jane's letters.