Per https://www.seat61.com/interrail-and-eurail-reservations.htm#Norway and vy.no, seat reservations are not needed on regional trains, so nothing needed to Hamar or Larvik/Skien. They’re available but not mandatory (I wouldn’t make them, esp. in September).
For Oslo - Myrdal book using the chat function on entur.no : no booking fee and directly sent by email. Free with a 1st class pass.
Thanks for the prompt response. Per the European Rail Timetable, the trains to Larvik/Skien do not appear to require reservations, and there are frequent trains so I’m guessing that they don’t sell out, but if there’s a good chance that they do I’d just as soon make the reservations.
Thanks, just saw your revised response!
Yup I've updated my post. DB, the planner I use regularly, wasn't exactly right for once.
Enjoy your trip !