There is a way for more oil to reach the carb because it is making more blowby or the valves are more worn. I don't know what normal is tho. I have a little oil in that tube also. Do you know if you have the original Blowby valve, sometimes called the Turkey valve, or the updated one? I think the test for blowby will be a leak down test. Sort of like a compression test but better. You could start with a compression test if you have one of those.
Air leaks are on the down side of the carb. Above the carb, on the side that goes to the air filter, dirt or grit can get inside but this is not a vacuum leak. On the side that connects to the cylinder head is a rubber attachment tube. These get cracked and stiff with age. They can be the source of a vacuum leak that effects how the machine runs. With bike running at normal temp spray WD-40 around the carb intake tubes to see if idle changes.
That plug looks like it is getting too rich fuel. It is washed on one side by raw gas and black from burnt gas on the other. Can't think of any other reason for it to look like that. Maybe it means something else to somebody else.
When was the last time you rebuilt these carbs? New O-rings and complete cleaning. If the diaphragms really do look good you can skip them. I don't think they cause high speed misses.
I think your main complaint is the high speed miss that seems to be getting worse? You say you have reset the points? This is generally not a good idea. Points do wear and in normal operation will have a hill formed on one point and a valley on the other point. This can not be set with a feeler gauge. It can be done with a dwell meter. At this point I install new points if I think I have an ignition problem.
If you want the ignition points to last use a Dyna Booster box.