Obligatory fishing report: Didn't get back from striper fishing until 3am this morning, but I caught two nice fish and we just ate them for dinner!
I worked on the 461 this afternoon, and have solved the mystery. First, I put on the 28" bar/chain like I always do. Normally, I put the chain on the bar, loop the chain around the drive sprocket, slide the bar on the studs, tidy up the chain, put the clutch cover on, then put the bar nuts on finger tight (just enough to press the bar flat and get everything into alignment). Using this approach, it's not possible to tension the chain all the way, and I was again convinced there was too much chain or not enough adjustment.
So I pulled the clutch cover off, intent on taking some pictures and making some more measurements. With the cover off, I was able to tension the chain no problem, and what's more, it only took about half the range of the tensioner. I had a WTF moment, sure that I was losing my mind, but I put the clutch cover back on, fine tuned tension, and everything was great. Ran the saw to break in the bar/chain, and it's all good.
I went back to see what the issue might have been, and near as I can tell, the bar tensioner slider just doesn't have a lot of slop -- any sort of interference and it binds. If you crank the screw in that situation, the pressure pushes the end bushing of the threaded tensioner shaft (the bushing is the white nylon part on the right of the original pictures) and that bulges out a little, against the tensioner cover plate. I had seen that bulging the other day (you can see it looking under the clutch cover along the bar) and associated it with running the tensioner to the max setting, since the same thing happens in that case. So the saw was never actually running out of adjustment. Rather, the tensioner was stuck and the act of turning the screw had the same symptoms.
Interestingly, this whole issue does not happen with the 20" bar, only the 28" bar. Maybe it's a little thicker (paint?), maybe the tensioner pin hole is a little off (both have a blob of paint in them), or maybe the extra bar weight leaning out there changes the fit a little. But whatever the case, it's not easy to tension with the clutch cover on and the nuts finger tight.
I may take another look at the tensioner and see if I can maybe squirt some bar oil in there and help it loosen up. I'll also drill the paint blobs out of the tensioner pin holes in the bar. I have a feeling this problem will go away with time, but for now, the easy solution is to just adjust bar tension with the clutch cover off. I used to do it that way anyhow.
Anyhow, thanks for all the input guys -- that definitely helped me realize that the specs were all in line, and something else had to be at play. As usual, the mechanical problem came down to the "nut" operating the saw.