1. To fans of Adaptability, why is it more pro-concept or more necessary than Rock Head?(and Sheer Force)
Adaptability is more pro-concept compared to Rock Head and Sheer Force as it literally takes the strengths of
both our typings and amplifies them, whereas the other two takes the offensive strengths of
one typing and render the other typing questionable, which I think is undesirable. Just like how Rock Head doesn't benefit Poison STABs at all; Stone Edge and Power Gem do not get a boost with Sheer Force. As Poison and Rock do have rather redundant neutral coverage together, there is a good chance that this would result in only a single STAB being run, with several non-STAB for better coverage. Adaptability does not face this problem as neutral coverage moves are practically as strong as resisted STAB moves, allowing you to easily just pick up both STABs.
Adaptability is also the only ability out of the 3 which boosts Power Gem, which means it is the only one which does not practically confine us to a physical inclination. On the special side, Power Gem is at a respectable 106 BP; Sludge Wave is basically a nuke at 126 BP with no drawbacks; Sludge Bomb at 120 if you want 20% more Poison rate; Venoshock is at a beastly 172 BP with TSpikes or simply after Sludge Bomb spam. On the physical side, Adaptability pushes Stone Edge to Draco Meteor levels while making Gunk Shot marginally more powerful than Head Smash. It also makes utility moves like Rock Tomb, Acid Spray and Clear Smog more powerful, which the other 2 abilities do not affect at all.
Adaptability (like water-absorbing abilities) is in fact entirely anti-concept. It entirely takes away the weakness of Rock/Poison, which is that it is walled by Steel (and weak to Water).
I find it very hard not to disagree with this. Sure Adaptability makes CAP 21 have stronger STABs, but at the end of the day it is still just a 1.33x boost, which is practically a Life Orb boost. It is certainly going to be slightly harder to wall, but unless CAP 21 gets an astronomical boost in its offensive stats upon Mega, it is not going to be that much harder to wall by Steels compared to Life Orb base.
4. Are Abilities suggested in the previous stage, such as Water Absorb, Water Veil, Drought, worth using on the Mega forme? Why?
I don't see how Water Veil is ever worth it as an ability over Regenerator to Mega evolve into, even considering the +100 BST. If I really want to make my Mega immune to burn, I will just drop a Lum Berry and keep Regenerator on base, and Guts/Flare Boost would be better anyway. Mega Evolving for a Water immunity is not too bad, but imo Storm Drain takes the cake from Water Absorb, in that it will have countersweeping potential by Mega Evolving. Irregardless, it is still too conditional, and I would rather just pair base with Ferrothorn or Seismitoad or something to deal with Waters. Drought on Mega is inferior to having it on base form imo, as it it loses a niche on being sun support.
5. Continue discussing the role of the Mega Ability and suggesting specific Abilities for our Pokemon to have.
I still believe an offensive ability is the way to go. Regenerator is basically the epitome of defensive abilities, which render quite a good number of abilities unworthy to Mega Evolve into. This also makes Sand Stream for Mega much less desirable since AV base pretty much does the same job unless the +100 stat is amazingly distributed. Regenerator as base ability also means simply slapping an Life Orb would outdo practically every conditional offensive ability. That taken into account, I would say the there are only a few other offensive abilities that are worth mentioning.
Hustle can work, but it means no reliable physical moves unless it comes with Coil or Hone Claws.
Moxie is another ability that might work. However, both these abilities have the same problem as RH/SF.