Yes, existing connections are kept as long as there is traffic often enough (in either direction) - the timeout is configurable.
This is truly amazing work, I'm glad you chose Nat as your NF example. One minor question I haven't seen answered anywhere: For vignat, in the git repository and the paper, I see no mention of software license.
May I assume that it is in the public domain, or if not, BSD licensed?
Good point. Will talk about it with the other authors. Thanks.