Eric,
I think I can suggest the optimum route now. The route immediately south of The Dalles on the Old Dufur Road is partly gravel...just so you know. I don't recall any roadside artifacts or bridges, but it is a pretty drive.
Turn east to Boyd and stop at the old mill and bridge (same site). The choice then is to go into Dufur or follow the old alignment south. Either is interesting. The old hotel at Dufur is worth a stop. On the other hand, there are a couple of old ABB landmarks (community center, abandoned farm house) from the teens along the old alignment. I guess the Dufur route (Boyd Loop) is faster and will also avoid some gravel on the old alignment.
Either way you are going to head next to Tygh Valley. Stop at the General Store or Cafe.
You can catch the bridge across the Deschutes at Maupin whether you go across Sherar's Bridge or directly southeast to Maupin. As far as old road alignments, it is no contest. The road from Tygh Valley to Sherar's Bridge (Oregon 216) is the winner. It is (1) the historic route, takes you past a (2) lovely water fall, under a (3) pioneer railroad trestle as you approach the river, lets you enjoy seeing (4) Native American fishermen fishing as their ancestors did, follows along the river (5) allowing you to watch the rafters, and when you get to Maupin, you can (6) detour 2 minutes to the bridge ( was Hunt's Ferry in the old days) before you head toward Bakeoven and Shaniko.
From Shaniko go south to Antelope, then west back to US97. Antelope became Rajneeshpuram when Bhagwan Shree Rajneesh and his followers "took over" the town in the 1980's. That is another story!
When you intersect 97 (Willowdale is across the road on the west side of US97) go about a mile north to the mouth of Cow Canyon and turn left (west) at the truck safety ramp. That puts you on the old Cow Canyon alignment. Travel up the canyon as far as you like. It is only about 3 miles to the top.
Come back down (south) on the modern highway, past Willowdale (just a farm complex today, but a major stage stop in the old days), and take the old alignment I identified earlier, perhaps all the way to Gates with its old train station (unrestored).
There is an abandoned house or two between Gates and Madras, but I don't recall anything spectacular. From Madras you can take the old alignment via Metolius and Culver. After you gaze into the Crooked River gorge and view the high bridges, you will note Smith Rocks off to the east, and soon enter Terrabonne
The old alignments south of Redmond can be interesting, but they are no longer pristine, so I won't detail them here.
I have included a 1915 map out of my collection
HERE. It is on a pretty large scale, but will clearly show the old roads when you look carefully.
I'll take a look at your other questions ASAP
Dave
Keep the Show on the Road!
PS
"One question about the Shaniko to 97 route. Route 216 will take me east and north to Grass Valley. Did the old road really do that? Or did it take Ball Lane to Finnegan Road to Kent?"
You don't want to go via Grass Valley. Turn south along the east side of the Deschutes River at Sherar's Bridge and go almost to Maupin. If you want to see the bridge at Maupin, detour the half mile or so to get there. In any event, take Bakoven Road from Maupin to Shaniko.
The only reason to take the much longer Oregon 216 to Grass Valley would be to see Kent. There is a great old abandoned service station there that still had the gas pumps a couple of years ago. But it is a long detour compared to taking Bakeoven Road.
If you wonder how folks got from Sherar's Bridge to Bakoven, they went up a long grade that is above you when you are driving south on the Deschutes River road. This old road comes out at the electric substation on the Bakeoven Road.