5. Does sun or sand need the primary ability?
As many people have said before, Sun is the weather that would benefit the most from a stronger ability like Drought, which makes it the best choice for the primary ability slot. However, I think there's one Sand ability that might des the slot too:
Sand Stream: The ability to automatically summon sandstorm would be ideal to support Excadrill, and it'd allows us to replace Tyranitar/Hippowdon as setters, while still having great type synergy with existing abusers thanks to our typing. It should also be noted the only notable Sun ability that would be excluded as a secondary ability is Drought, meaning that if we were to choose Sand Stream, there would still be other powerful choices available to support Sun, such as Chlorophyll. Of course, this doesn't mean that this ability is not without some serious problems, for starters, it does not provide the user with a sand immunity meaning that CAP 24 would be forced to take sand damage. Not only that, one might argue that by just using CAP 24 as a Sand setter, we're not really abusing it, instead, it would be relegated to a support role on a sand team, simply providing the appropiate weather for others to use, but never actually taking advantage of that weather by ourselves. Personally this is not my favorite choice for a slot, but I still believe that it is at least worth considering.
As for my thought on other abilities:
Drought: This one has gained a lot traction, and its easy to see why. While it doesn't provide Chlorophyll's speed, just the ability to set up Sun without depending on the mediocre Malaconda is huge, and it would boost Sun more than any other choice. That being said I find the part of snake's argument about focusing on wallbreaking instead of speed control to be questionable at least. There a reason why we chose to focus on the later: Sun already has all the wallbreaking power it could ever want, Specs Volkraken is already one of the best Rain abusers, and under the Sun, it is arguably even more dangerous thanks to Fire Blast slightly higher accuracy when compared with Hydro Pump, and more importantly, the lack of any viable Fire-immunities outside of Heatran (which can still be 2HKO by Hydro Pump even under the sun: 252+ SpA Choice Specs Volkraken Hydro Pump vs. 248 HP / 220+ SpD Heatran in Sun: 216-254 (56.1 - 65.9%) -- guaranteed 2HKO after Leftovers recovery), and even Arghonaut, the only Water-Type capable of taking 2 Sun-boosted Fire Blasts on the switch with Stealth Rock on the field, can be beaten if it takes even just a tiny bit of damage before (something that Toxic Heatran could do very easily). While it is true that we could simply leave the speed control role to another teammate, it would still be much better if we were able to provide this by ourselves, as there are still many ways this could be done even without having Chlorophyll as our Sun ability, as providing yet another wallbreaker would be completely redundant. However, we should not forget that auto-weather abilities are some of the strongest ones available, and it could end up overshadowing the secondary pick quite easily, meaning that we'll have to work extra hard to prevent that from happening.
Chlorophyll: This is clearly the best ability for providing Sun teams the speed control they so desperately needs. Of course its main drawback is that we would still need to depend on a preexisting setter, which are all pretty underwhelming. Despite this, the power that a good Chlorophyll user would possess should not be underestimated, as the speed boost will could remove almost any offensive counter-play against CAP 24 as long as the Sun shines. Personally, this is my favorite pick, because it allows us to abuse Sun the best, and if we are sufficiently strong, then the only thing Malaconda will need to do is switch-in 2 or 3 times in a game, and just let CAP 24, Volkraken and Heatran do the rest of the job.
Sand Force: On one hand, I have to agree with DLC, boosting just one relevant type (Ground), it is a underwhelming ability. However, I still believe it is probably the best choice for a Sand-based ability, because it allows us to deal more damage to stuff like Toxapex, M-Crucibelle and Heatran, and most important of all, gives us a real incentive for running CAP 24 on a Sand team.
Sand Rush: It is certainly stronger than Sand Force, but its kinda redundant, as Sand already has a decent user with Excadrill. However, just the fact that it directly bounds us to use Sand makes it a far superior choice to any other ability.
Finally I'd like to answer to people that think this concept can work without picking an ability directly linked to weather. The problem with doing that is that without an appropiate ability, the only thing that connects us to weather is our supposed synergy with pre-existing abusers, that reasoning has already failed to accomplish several concepts in the past (Voodoom, Volkraken, Plasmanta) . Trying to boost the viability of a mediocre Pokémon by introducing a supposed "partner" has never worked in the past and I see no reason why this time will be any different. People have brought up that Rain is able to use mons that don't directly benefit from it, such as Hawlucha, but they ignore the fact that Rain is able to stand on its own, that is why stuff like Lucha can happily patch up holes on it. On the other hand neither Sun nor Sand are able to accomplish even that, and are only a burden for the rest of their team. Tapu Bulu, a Grass/Fairy type that complements some of their worst weaknesses already exist, and yet, I have yet to see a successful San/Sun team with it (I am aware that Bulu it's unable to beat Tomohawk, one of the biggest threats to Sand, but it still can support its team not only by resisting Ground, but also by removing Excadrill and Tyranitar weakness to it too) [EDIT: reachzero pointed out on Discord that I made a dumb mistake here, as Bulu halves the power of Excadrill's Earthquake, so this is a bad example]. We can also still accomplish our goal of having use outside of weather without the need to have a competitive ability, there are multiple mons in OU with middling or outright useless abilities such as Zygarde, Mew, Zapdos, Keldeo, Volcarona, etc. Of course this doesn't mean that weather abilities will guarantee the success of this project, but I think they will have a decent chance at succeeding, unlike the other option, which we should know by now will be doomed to fail.