In St Regis the trail does not start until Little Joe Creek Road for the Route of the Olympian. When you say the route east of Tarkiro - I think you are referring to Tekoa, and that detour is well documented, as well as the one around the private land with the missing trestle.
I am taking about Tarkiro/Triple Bridges and the section a bit west of there that hugs the river appears to be a private road:
https://www.google.com/maps/place/Triple+Bridges+-+Alberton+Gorge/@47.0200808,-114.6613939,16z/data=!4m5!3m4!1s0x535e0d62dec45c27:0x1ea873ae07d87b79!8m2!3d47.0200772!4d-114.6570166And I rode the first couple of miles west from St. Regis in 2017. It was horribly bumpy. I was bouncing around on 37c tires. Once I reached the point to detour around the missing bridge I opted for I-90. Note that I also encountered a motorist. The NorPac Trail is also open to motor vehicles. The first time I rode the west slope to Lookout Pass I encountered one car and later two people on ATVs.
All that aside, one thing you might want to consider is incorporating the Route of the Hiawatha Trail. Really cool ride which I did in 2019. Since I was heading west, I took Little Joe Rd. all the way to what I was later told is called Gold Pass at the border with ID. 15 miles of gravel, but there are only a couple of sections that are tough (8%+). At the pass/border, the road becomes paved. Appeared to have been resurfaced relatively recently. The descent is fabulous and quite scenic. Once you get down to the St. Joe River there are a couple of campgrounds with water before Avery. I stayed at Tin Pan Flats. Virtually no traffic on a Sunday. The next morning I rode into Avery and took the old Milwaukee Road right of way up to the Pearson Trail Head of the Hiawatha. It is also gravel and features a few easy tunnels. The Hiawatha Trail has about a dozen more tunnels and several high trestles. The last tunnel heading "east" is the St. Paul Pass Tunnel, which is 1.66 miles long as brings you back into Montana to the East Portal Trailhead. A good light is required as the tunnel is completely unlit. I had 500 lumens. Minimum recommendation is 350 lumens. From East Portal it's easy to connect up with the NorPac Trail.
This is the map for the day along the St. Joe from a bit west of St. Regis:
https://ridewithgps.com/routes/30007721Then the day to Wallace, ID that included the Hiawatha Trail:
https://ridewithgps.com/routes/28822925Ignore those crazy elevation spikes along the trail. GPS is reading off the tops of all the tunnels. The spoke at 42.9 is the one tunnell on the NorPac Trail.