mmm I'm confused - aren't the attacks essentially separate (both start from the declare attack stage), so the flying creature can lose flying when attacking the ground creature (see point below where I ask this better I think).
Laddinfance uses this example in his clarification you link to:
Laddin: Also, in order for a Flying creature with a Sweeping Melee attack with reach to attack both Flying and non-Flying targets then they will have to lose Flying (presumably by attacking the non-Flying creature first).
He does agree with you later though that:
You: if a flying creature with reach attacks another flying creature, they would not be able to make a secondary attack (sweeping or battle fury) against a non-flying creature, correct?
Laddin: Yes
It seems like the Flying reach sweeping attack ground 1st, air 2nd works with the rules as stated (although as you note, probably doesnt need stating). Creature attacks to ground, loses flying, but has reach to allow second attack on flying creature.
The reverse seems new, but doesn't seem to break other rules or be contradictory with the other posts:
flying reach sweeping attacks air 1st (allowable) ...
actually perhaps this is the point you're debating - on page 2 other thread you state:
Therefore, if your first attack is against a flying creature, none of your additional attacks can be against non-flying creatures because you can not give up the flying trait to land and hit them.
Why not? I understand sweeping ... ok Codex says, A sweeping attack is one attack action with two attacks...
Can you not loose flying during an attack or an attack action? Laddinfance's example above suggests you can..?
Essentially the clarification means normally sweep happens on the same plane (air to air+air, air to ground+ground). Sweeping has clarified that reach can break this, but hasn't changed other rules. As you say, air to ground + air is already allowed, but air to air+ground wouldn't have been.
Reach has to break the other rules in order to function that way though. If a flying creature wants to make a sweeping reach attack against first a flying creature and then a non-flying creature, it will have to attack the non-flyer while it itself is still flying. A creature can not gain or lose flying in the middle of an attack, and thus it can not land to hit the non-flyer like it normally would have if the non-flyer had been the first target. Therefore, either it can't hit the non-flyer, or Reach would allow a flying creature to hit a non-flyer without landing (thus preventing counterattacks). If Reach were to allow such an interaction with Sweeping, it would also then open the question why it can't do so with a non-Sweeping attack.
This has actually already been discussed in great detail. See >>>here.<<< I'm just double checking that they haven't changed their mind on it, because the updated codex still creates confusion regarding the matter.