Also, stun is a best-case status effect for your argument.
I gave several others as well, such as immobilizing defenders and dazing strikers. I'd hope I wouldn't give a worst case example, hmm?
The length of the combat is
not fixed, and, in fact, is affected (even effected

) by the outcome of various attacks.
Of course it is, but which of the two characters gets the buff and gets stunned two times more doesn't necessarily change the duration of the fight by an entire round. In fact, given that the high defense character was down to at-wills, the only real possibility to shorten it is if the low defense character gets the bonus, so can use encounter powers that have a stronger effect.
Now, in actual combat with a party, this solo is going to take out the guy that he can hit so easily very quickly
Hardly a given - the stun attack might not even do any damage, but the damaging attack be against a different defense entirely. A dracolich, for instance, can easily take that long to kill and stun a ton and a half of people, but only deal 16 necrotic damage with its breath and none with its glare. Against a character with 10 necrotic resistance, it might not even require a single heal, nevermind one of the three to six the party might have available.
Additionally, stunned is a really exceptional effect here, being one of the least clear-cut cases.
_Least_? It's one of the most clear as to what happens with it - all of the rest are extremely subjective.
If the effects include things such as, say, forced movement (throw em into the ravine over there) or any other primarily offensive effect, it's again far more clear-cut in favor of raising high-defenses.
You still haven't actually shown that. If you don't like stun for some reason, try immobilizing or restraining melee characters out of melee, or ranged characters into melee, or dazing the ranger, warlock, fighter, or swordmage.
As I said before, effects muddy the waters
And yet, they're the most important aspect of any discussion of FRWs being too low.
If you pay attention to damage, you'll get incorrect answers like:
1) Well, PCs aren't taking enough damage now, so improving their defenses is bad
2) If a creature deals too much damage at 20 damage per hit with a 95% hit rate, reduce its damage to 12 instead.
The first is still bad, from a perspective of effects, fun, and fighting futility and frustration.
The second is still bad, because even if you address the average damage you still need to address all those other things.
relying on character-optimization to choose to raise the lowest defense and help balance things is unwise.
Absolutely agreed.
Hence my suggestions to raise the NAD's (particularly the low nad too) as part of the global game levelling mechanics, and not as part of the character customization-choice via feats. Feats just work really poorly for this.
Again, agreed.
You do need to be careful, though - a character might have +1 to +3 from an item, +1 to +3 from masterwork light armor, +6 from feats, under core rules. So any house rules need to potentially adjust to that.