Neither of what you said is true unfortunately. Positional Advantage has nothing to do with weapons that have the Light property even in the *literally never used anywhere ever* optional rule of "Facing" in the DMG.
And if you think it makes sense, what is your response to the 56+ class abilities and spells that are completely invalidated by the two homebrewed systems?
The rule exists page 252 black on white in print. You can't blame them for making use of it and limiting it a tad.
As for second point - citation needed. "Invalidating" is a BIG word there, I'd imagine the actual truth is somewhat not there. Overall the point is that there will be plenty of changes and in my opinion many of these are deliberate changes to make the game less frustrating for people who do not necessarily get off having RNG miss streaks. I'm down with them tampering with AC values and adding a few extra sources of Advantage for that, it won't detract from my enjoyment of BG3 and likely will only increase it by reducing wasted actions.
If there will be spells that will be legitimately useless given these changes, then they either will be just that or will be amended (like Sleep, for example, btw). It's not unlike BG/BG2 - you also had a ton of useless spells there and these games were far from faithful 2e recreation - somehow the world did not end there and everyone and their mothers put these games on a pedestal as a model D&D videogame.