High Elf commoners drafted into service serve as either Archers or Spearmen depending on experience, High Elf nobles serve as Silver Helms or Commanders, depending on experience. This is why these are the common types of units fielded by the High Elves - because ALL their armies have access to them, in considerable numbers. Hence Core.
Province-specific units are always deployed in considerably lower numbers, or not at all, hence why they are special choices.
Dragon Princes should be rares, due to... well... being bloody rare. They are formed only out of the creme-of-the-crop of the nobility of a single province... that's not a whole lot of Dragon Princes to go around.
Ellyrian Reavers being scouts is a non-issue. Most armies may have scouts, but most settle for just that - scouts. You don't have to give them excessive training in mounted combat using multiple weapons and form them up in entire units in order to use them as scouts.
In fact, I'd argue that's rather counterproductive... :P
Those armies who don't have access to Reavers are simply going to have to make due with their own scouts, whether these are Shadow Warriors, specially trained individuals who don't fight alongside the army, or simply skilled citizen-soldiers on a special assignment.
Ellyrian Reavers are just as rare as the other province-specific units, and as such should remain special choices. In fact, I wouldn't complain about Lothern Sea Guard getting beefed up to special status, either.
Removing SoA is only going to "break" them if we assume the cost(and everything else, as well) stays the same. Yes, for 15 points, they'd be crap, but for 12-13?
As mentioned, the Greatswords are doing fine on 11(10?), with a point more of armour. White Lions on their end have +1 armour vs shooting, as well as more skill and strength. I'm no master of mathhammer, but I'd still consider taking them at 12-13 points(if they hadn't been against the theme of my army, anyway :P).