The Mega-Diancie set is an utter trainwreck. How does Rock Polish logically mesh with a Defensive spread. Diancie should only be running tank sets or full out speed sets. Furthermore, Diancie should be running max SpA and either max Spe or max HP. Hasty for the former and Quiet for the latter.
As for the Altaria, you DO NOT need Dragon STAB. Fairy + Ground is more than sufficient for its purposes of sweeping. The Dragon STAB is superfluous and useless.
How... how exactly is that a defensive set? I went with HP and Speed investment to help Diancie survive a surprise Bullet Punch from a non-STAB user of it, and because 160 base Att and SpAtt is high enough that it'll deal out huge chunks of damage even uninvested. I'd have to calc things out, but I remember a lot of Bullet Punchers dealing upper 90's max damage with it while Mega Diancie could get at least a 2HKO with either Diamond Storm or Moonblast, so while you wouldn't be switching Mega Diancie into them, they'd need to sacrifice their Bullet Puncher to Mega Diancie and hope to finish it with another priority user. This wouldn't be possible without the HP investment. However, I might be wrong, and as far as I can tell Smogon's damage calc is down, so I'll have to do the calcs manually and get back to you.
As for Mega Altaria, I already mentioned above that I realized that as I was typing it up. The Dragon move is only included in the event of a pokemon that would take no more than neutral damage from Ground and is resistant to Fairy, since very few of them also resist Dragon (pretty much limited to Skarmory, Levitating Bronzong, Forretress, Ferrothorn, and bulky Steels with an Air Balloon or Magnet Rise). Given that it has decent SpAtt uninvested, event with Adamant nature, Flamethrower could be enough to cover those threats with the exception of Air Balloon Heatran, and most others pokemon I didn't mention with similar resistances don't have the bulk to even handle an EQ or Return and/or don't have the ability to deal out enough damage to overcome Altaria's own defenses. At the end of the day, though, you're right in that it's not really needed, since its use is limited to a few specific threats.
----- (Edit added below) -----
Alrighty, so. First thing I have to say is that I never even considered Earth Power as a possibility solely because at the time I typed those sets up, I didn't even know it was a thing for Diancie. This makes a SpAtt/Speed or HP/SpAtt spread much more viable for it, since it can run a pure Special set with Moonblast and Earth Power, and such a set would also have a considerably stronger reason to use HP Fire since now it can pack a much stronger punch with it. That said, I ran calcs regarding the two pokemon I'd say are Mega Diancie's biggest threats: Mega Lucario and Mega Scizor. I took into consideration Mega Diancie running either neutral nature uninvested or positive nature fully invested in the appropriate Attack stat and the same for the appropriate Defense stats for both of the above named threats (because hey, you never know if someone is gonna think a fully SpDef Scizor is a good idea ) Here's what I got for the results:
0 SpAtt Moonblast vs 0 SpDef Mega Lucario - 217-256
252+ SpAtt Moonblast vs 0 SpDef Mega Lucario - 280-330
0 SpAtt Moonblast vs 252+ SpDef Mega Lucario - 140-165
252+ SpAtt Moonblast vs 252+ SpDef Mega Lucario - 181-213
0 SpAtt Earth Power vs 0 SpDef Mega Lucario - 275-324
252+ SpAtt Earth Power vs 0 SpDef Mega Lucario - 355-418
0 SpAtt Earth Power vs 252+ SpDef Mega Lucario - 177-209
252+ SpAtt Earth Power vs 252+ SpDef Mega Lucario - 228-269
0 Att Diamond Storm vs 0 Def Mega Scizor - 122-144
252+ Att Diamond Storm vs 0 Def Mega Scizor - 158-186
0 Att Diamond Storm vs 252+ Def Mega Scizor - 93-110
252+ Att Diamond Storm vs 252+ Def Mega Scizor - 120-142
0 SpAtt Earth Power vs 0 SpDef Mega Scizor - 98-116
252+ SpAtt Earth Power vs 0 SpDef Mega Scizor - 126-149
0 SpAtt Earth Power vs 252+ SpDef Mega Scizor - 71-84
252+ SpAtt Earth Power vs 252+ SpDef Mega Scizor - 91-108
0 SpAtt HP Fire vs 0 SpDef Mega Scizor - 265-312
252+ SpAtt HP Fire vs 0 SpDef Mega Scizor - 340-400
0 SpAtt HP Fire vs 252+ SpDef Mega Scizor - 192-226
252+ SpAtt HP Fire vs 252+ SpDef Mega Scizor - 246-290
Keep in mind that both Mega Lucario and Mega Scizor get the same base HP, so each gets a max of 281 HP uninvested and 344 fully invested. This means Earth Power is an amazing weapon against Mega Lucario to take place of neutral-hitting Moonblasts and the considerably weaker HP Fire that used to be Diancie's only anti-Steel option. While Moonblast is only a guaranteed OHKO on Mega Luke if Diancie is fully invested, it still has a small chance to OHKO so long as Luke doesn't have any SpDef investment. If it has HP investment, though, Moonblast is no longer an option at all, but to the best of my knowledge, defensive Mega Lucario isn't exactly a thing. Either way, though, Earth Power is better, trading STAB for being SE on him, with SpAtt invested Diancie guaranteeing an OHKO on him even if he's fully HP invested so long as he ignores SpDef, and if he ignores both HP and SpDef, Mega Diancie more than likely OHKOs even without any SpAtt, so long as she isn't negative nature.
In Mega Scizor's case, HP Fire is the clear frontrunner damage-wise, but is ever so slightly less powerful than Earth Power on Mega Luke thanks to it having better SpDef and HP Fire being weaker overall, somewhat mitigating the 4x weakness. Still, similar rules apply; if it isn't invested in HP, Mega Diancie OHKOs with HP Fire regardless in most cases, while an HP invested one still loses to fully-invested Mega Diancie unless the RNG gods decide to give you minimum damage. If both parties are fully invested in SpAtt and SpDef respectively, it's a similar situation, except only counting if Mega Scizor isn't HP invested as well.
Conclusion: So long as you don't roll minimum damage, chances are pretty good Mega Diancie can OHKO one or the other if she has at least some SpAtt investment.
So we have our ideal moves: Earth Power against Mega Luke, HP Fire against Mega Scizor. But none of that even matters if Mega Diancie doesn't survive their Bullet Punches does it?
0 Att Mega Lucario Bullet Punch vs 0 Def - 304-358
252+ Att Mega Lucario Bullet Punch vs 0 Def - 394-464
0 Att Mega Lucario Bullet Punch vs 252+ Def - 226-266
252+ Att Mega Lucario Bullet Punch vs 252+ Def - 291-343
0 Att Mega Scizor Bullet Punch vs 0 Def - 234-276
252+ Att Mega Scizor Bullet Punch vs 0 Def - 302-356
0 Att Mega Scizor Bullet Punch vs 252+ Def - 174-205
252+ Att Mega Scizor Bullet Punch vs 252+ Def - 224-264
Between the two, Mega Luke's is the stronger, but that doesn't really make Mega Scizor a lesser threat. Mega Diancie's base HP is 50 though, so it only has 241 HP uninvested, or 304 fully invested. Against a Mega Scizor, using my above set, you might survive on one or two HP through a Bullet Punch, but chances are slim without a Def buff. Against Mega Scizor with even a small amount of Att investment (and chances are pretty damned good it'll be fully invested) you'll need some Def to survive a hit. This is even more true against Mega Luke - you will not survive AT ALL if you don't have Def, even if you have full HP and he has no Att. You'll fare alright against it if you use a physically defensive, full HP Mega Diancie against a special set that has Bullet Punch, but chances are such a set would prefer Vacuum Wave for priority, and if it's also fully Speed invested, would wreck you with a Flash Cannon before you could move anyway.
So the key here is prediction. You'll have to limit the opponent's options and force them to switch into Mega Luke or Mega Scizor while you have Diancie out, and use the appropriate move to take them down. This actually proves the point you made, Eaglehawk: regardless of other investment, Mega Diancie is much better off running full SpAtt and a SpAtt buffing nature (most likely Rash, since you need the Def more than the SpDef). This makes Diamond Storm more or less pointless, since the goal is to kill either one on the switch-in, which summarily makes HP a little less useful as well. I'd say pick something you'd want Diancie to outrun and put just enough into Speed for that, then dump the remainder into HP. Moonblast, Earth Power, and HP Fire would be required for this kind of set, and you could choose between Stealth Rock if you don't have anything else to lay them up, Diamond Storm to help handle Flying types and potentially buff its defenses, or Rock polish to make for a decent late-game Special sweeper.