I'm not going to be able to answer this to anyone's satisfaction, but I started checking the oil level when hot (after roughly a ten-mile ride), rather than a two-minute idle like before, and now judge that as being the real oil level (per the advice of several people on this forum). When I first did this, of course, the oil level showed well over the FULL line, so I drained it down even with the mark. I continued to check the oil level when hot (at FULL mark), when stone cold (1/4th inch below FULL mark), and after a two-minute idle (dropped down halfway between the two marks or lower-can't remember). After doing this numerous times and seeing that the levels were consistent at each of these intervals, and that the oil pressure was good, I decided I was no longer getting gas in my crankcase, and that the oil pump was working fine, and that the high readings I was getting were due to me not knowing when to check the level.
I have to reiterate, though, that I was originally getting gas in my crankcase, and that my overflow tubes never worked (I made sure they were clear). And it's still a mystery to me why that happened, except for something someone mentioned earlier, which is that when you've got excess fuel in the bowl, it seems more likely that the vacuum from the engine will suck it through the carbs rather than allow it to drain out the overflows. I would love to see a more thorough debate on this issue, if anyone's game. And I've got some other half-baked ideas I'd love to get some feedback on, so I don't spread anymore misinformation than I already have.
Thanks!
Steve.