The Tongariro Crossing is definitely not easy, not just the uphill parts but also the downhill for those of us with aging knees. But you can do the first sections from either end. From the south, the walk to Soda Springs is pretty flat.

More generally, I figure that if you have already spent a month in NZ on a previous trip, you have a good feeling for the place and your own preferences for nights spent in a place etc. So overall your draft is doable in my view. Some specific comments follow that may or may not be useful.

If for some reason you needed to rejig or shorten the trip, deleting the trip down to Wellington and back would save a lot of time. That said, if you do get to Wellington, the bird sanctuary Zealandia is well worth a visit. It's can be reached in just a few minutes by car, or by the cable car to Kelburn and a 20-30 minute walk. If you happen to be in Wellington on a Sunday, there is a good farmers market on the waterfront near the Te Papa museum, with lots of food stalls.

One place on your draft itinerary that I thought could warrant an extra night is the Coromandel, even though it is not so large. It is the only beachy part of your trip and there are some good ones, including a hot water one. Looking at where you might steal a night from somewhere else, I would contemplate Taupo. It is a relatively short drive from Napier, so you could get there quite early in the day. And while it is a fine place with some decent restaurants you could get by with just a night if you wanted.

Rotorua has a lot of activities aimed at tourists. Most I can take or leave, so I leave that for you to decide. But I would recommend the guided tour at the Whakarewarewa Village (and thermal area), together with the cultural show that happens every day around noon, especially if you have not experienced Maori singing and dancing before. In general I find the restaurant options in Rotorua mediocre, so I don't aim high.

If you wanted to to go really off the beaten track you could go from Napier to Rotorua via Lake Waikaremoana. The lake is beautiful (weather permitting--not guaranteed) with a nice national park visitor's center and a motor camp with cabins, and some nice walking The road is not as terrible as you might imagine from the online discussions, but it is narrow, windy, and unsealed gravel for 50 miles or so. Some rental car companies may not permit it.

That's it from me. Hopefully some others will chip in.