I dion't see why someone cannot be both. In fact I think it is quite likely that several people would be represented on both lists.
As to adding the fourth criteria. I wouldn't be in favour of that for a very simple reason. I think it would exclude people who are motivated to teach and have been found to be helpful in explaining things to others but who might pitch their explanations at a particular level.
E.g. Someone who pitches their explanations at a newbie level - Sardaukar for example - does provide a very useful service and idoes, IMO, warrant a nomination as Sensei. On the other hand his explanations will be of little use to more advanced players.
The situation is even worse when you are talking about someone pitching explanations of advanced concepts. This isn't a paid lecturing job so some element of pitching the points made at a particular level and accepting that it may be a year or two of play before others have enough basic and intermediate knowledge to begin really using those points in-game must, IMO, be accepted. I don't buy into the thing that all lessons can appeal equally to all levels of students at all times. I think that's just far too PC and unrealistic. Sure you have to bring weaker students along but at some point you have to recognise that to understand certain points you need to understand certain fundamentals and thus some "streaming" is important. If that wasn't the case then you'd just be able to lump all 4 years of a degree course in together and let them go to whatever lectures they wanted whenever they wanted so long as they went to them all in the four years. That, obviously, isn't feasible or even commonsensical.
So, I think that fourth criteria is not a great idea on the basis that it would create what I can only assume is this unintended consequence.
Aye, I think that when one player is very good at certain things it often serves to make the other's play look "bad" when, really, that is merely a reflection of how good the other player's play is. Of course the converse can also occur. I think the former is happening in your game vs AndyMac while the latter is, possibly, happening in Greyjoy's game.
John Dillworth: "I had GreyJoy check my spelling and he said it was fine."
Well, that's that settled then.