I completely agree with you here, DougF. I don't know this for fact, I am only speculating, but my thought is that in the very early days of the game before the modern rules had time to evolve, the word "snooker" probably did mean that the ball on could not be contacted directly; you would either need to swerve or play cushion first to contact. The early rules probably did not define it at all because they would not have needed to do so. It was just a term that everyone was just assumed to understand. In a similar way, there is no definition for the term "fluke" anywhere in the Rules today. The word is not used at all anywhere within the text. It simply is not needed for any Rules clarification purposes so it is not there. At the beginning, I believe it would have been the same for "snooker" because everyone knew what it meant and it wasn't needed for the purpose of the Rules, it was just the name of the game.
There was always a penalty for failing to contact the Ball On from the very, very beginning. (The 1890's is the earliest written reference I found for this.) But in those ancient times (ancient as far as Snooker is concerned), the penalty for what we call a "miss foul" today was simply the value of the Ball On, as low as a single point for Red. At some point, the governing authority would realize that players may well decide that giving up such low value penalties was worth doing in order to be sure to NOT make contact and possibly develop a ball for the opponent to pot. So at some point (early 1900's by my reckoning but not certain), the minimum penalty value was made to be four points so it became more costly to purposely foul this way. Eventually (don't know when), someone came up with the idea of Free Ball as a more severe punishment in the event of a miss foul. And thus, it became NECESSARY to define, "Well, what is a 'snooker' actually?"
So now that it was needed in the Rules, they could have defined it as being unable to make contact directly because the Ball On is completely hidden behind the snookering ball. (Remember, this is the common definition that I believe would have been understood at the time.) But there is a problem with that, namely, that if a Free Ball cannot be invoked until the Ball On is so difficult to hit because it is fully covered by the snookering ball, then why would anyone ever actually play a Free Ball at all? When this situation would occur, you may as well just go ahead and put your opponent in to play at the Ball On again because he is obviously faced with a very difficult stroke being hidden like this. (Hey, he is "old school" snookered, right? He just might foul again! By the way, "play again" as an option also goes way, way back to the very early days of the Rules.) So the point is, if that were decided as the definition of "snooker", then no one would ever take the Free Ball option, they would choose instead to make the fouling player play again. So Free Ball would not have been a very punitive rule and not very useful. So instead (again remember, this is entirely my own speculation), they re-defined the "official" definition of "snooker" to be different than what everyone previously knew (but wasn't written down anywhere) and they came up with the current definition to go hand-in-hand with this newfangled "Free Ball" Rule.
And really, it makes perfect sense. From a practicality standpoint, there really is no problem at all either. So for your example, all one really needs to do is to add a single, descriptive adjective and the entire thing is made perfectly clear. So if you played at Yellow trying to trap your opponent, the commentator may say something like, "The attempt resulted in a partial snooker but the incoming striker can easily see a quarter of the left hand side of the Yellow." You would understand that result even if you were listening on radio. And if it were a true, old time-y "snooker" like I described above? Well, that is simply referred to as a "full snooker" if you are saying it as a noun. Or you can say the incoming striker is "fully snookered" using the same term as a verb. It is clear as a bell. A "full" snooker WILL require a swerve or an indirect stroke playing White off cushion.
I agree with your premise but certainly, there need not be any contradiction or confusion.