No such thing as invaders here. Everyone is welcome and encouraged to post! As for your questions:
-Nobody knows yet, but the Bungie stream at 2pm EST should answer some questions. I have to assume the new social areas will be mostly or completely open regardless of level. Pure speculation though.
-Absolutely balanced for one character. Many people have a happy, healthy Destiny relationship with that one special someone. Like you said, your drop chances will be limited but so will your expenses for leveling up gear.
-Common misconception about Destiny's Light Leveling system is that it's comparable to a traditional RPG where over-leveling is possible. In Destiny, your weapon damage does not increase whatsoever if your character's level is higher than the enemies level (e.g. going from Lv30 to Lv31 will not do more damage vs. Lv30 enemies). Only higher Attack rated weapons do more damage, e.g. a 331 Atk GHorn does more damage to all enemies than a 300 Atk GHorn.
Increasing light level will slightly reduce the incoming damage you take but only because of the higher Defense ratings on higher Light armor. Not a huge difference. On the flip side, being a single level below the enemies gives a huge ~32% penalty to both damage dealt and received. So reaching the same level as enemies in a raid makes an enormous difference but going above their level is negligibly important.
Light leveling might be the worst RPG-leveling system I've played. Aggressively counter-intuitive.
