I was finally able to take some time to work on the bike today.
I replaced the countershaft oil seal, the sprocket spacer O-ring, and the clutch pushrod oil seal. It turned out to be a 8:30am to 2pm job, including a half-hour gap to bicycle to the auto parts store for supplies.
The biggest issue was getting to the third 10mm bolt on the oil seal retainer plate. It was too close to the crankcase to get a ratchet socket or the closed end of a mini-wrench on it, and due to the angle forced by the surrounding parts, I couldn't get the open end of the mini-wrench on it solidly enough to break it free without stripping it. I also didn't feel like manufacturing my own tool for it like the Black Lab Adventures guy did. But I did notice that I could get a ratchet socket on it if I removed the magneto cover from the crankcase, so I drained all the oil from the engine and did that. When the time came to put everything back together, I just skipped that problematic bolt and bent the soft steel oil seal retaining plate so that it placed a little tension on the pushrod oil seal area when secured only by the other two bolts. If the absence of the bolt does cause a problem, I'll just Dremel a flathead slot into the bolt and use that to reinstall the bolt.
The other difficult part was getting the old oil seals out. I tried prying them out with a flathead screwdriver, but that did absolutely nothing. I found a hose-puller tool that I had previously made from a bent metal rod, and then I bent another hook into the other end with as small of a radius as possible. The clutch pushrod oil seal then came out relatively easily, needing only four attempts with my tool before popping free. The countershaft oil seal was much more tenacious, needing fifteen minutes of hard pulling, including re-bending my tool twice (since the force needed was straightening out the hook).
I didn't see any obvious damage to any of the seals, except for a little crud on/in the sprocket spacer O-ring. Installing the new seals was quick and easy. Rather than making a tubular seal-pusher tool, I just grabbed a 1/2"-drive extension bar and used a rubber mallet on it to tap the seal in using a circular series of taps until each seal was in flat and at a sufficient depth.
Reinstalling the clutch screw assembly resulted in the clutch lever failing to disengage the clutch. I had to go back to the service manual to figure out how to put it back into adjustment. As far as I can tell, the dealer service department didn't re-seat the clutch cable at the handlebar properly the last time they worked on the bike, and my removing the clutch screw assembly relieved enough cable tension for it to return to its proper position (necessitating my pushrod contact adjustment at the clutch screw assembly). Still, trying to figure out why reinstalling the clutch screw assembly exactly as it was before resulted in the clutch not working just added more time to the job.
Anyway, I finally fired the bike up, took it for a test-ride around the neighborhood, and couldn't detect any leaks. Then I replaced the sprocket cover and rode out to get lunch. No leaks or problems so far! I'll keep this thread updated if any problems do pop up.