1. Cthulhu/SSR Nike I guess? Ryu-Oh and Aphrodite too, as sanah said..
2. advantages of a water team? well, they are extremely buff and debuff friendly unlike thunder or wind.. or dark in terms of B frame debuffs... disadvantages... thunder events lol, water are really OP.. i don't think they have much disadvantages other than the obvious one and the fact that we lack fire events until rematches/adramelec / advantages of dark team.. well, they have no weakness like light, and the kami are great.. too bad you don't have dark amaterasu, though I probably have a few accounts with her, but no real nice combo.. if you want anyway.. disadvantages are just that dark ama is limited, ren is SR and not that strong, but gives -15% atk and def debuff in the same frame .. or you can always use sniper shot, but that defeats the purpose.. no other ex ability .. though, as you know, thanatos does have frame C def debuff -20% , so B frame may not even be needed much.. say if you have Satan and Thanatos, that's still 35% .. but if you just want that extra dmg then.. well, water seems like a better option tbh
water definitely does more damage with the right debuffs.. though it caps at -50%

3. next miracle ticket should come out around the same time SSR Cybele is out? I think? maybe it's in sanahs toolbox... not sure, but before Thanatos for sure..
4. nope... just d'arts sniper shot (though, in the future we do get special weapons for souls that give an extra kick to their abilities.. for example, D'arts sniper shot can become B frame ATK/DEF debuff -20% D frame ATK/DEF debuff -10% so -30% altogether.. but she can be used for any team, so it's not like a real substitute for dark in particular..)

5. Satan (Awakened), Thanatos, Susanoo (Awakened), Hades (Awakened) I guess? with Arthur/Morgan/Shingen as main soul ..
(in the future, Morgan will be buffed to give ALL allies rampage instead of just 1, and her buffs I think are to all allies? not sure.. but if so, she's incredibly OP)
(Shingen's burst ability is also an ex ability, so can be used if you have a soul like D'art)
Arthur of course for her 20% A frame atk buff, since no others give it...

Osiris can be a good substitute for Susanoo though, if she's not awakened, or if you really need her..
DMMs later dark kamis can be nice too, but i'm not sure of their abilities
I know that Berith is good, she applies up to 150% ATK buff to herself, plus applying rampage which pretty much double/triples her attack, intercept which does the same and attention to self (which is similar to taunt), guaranteeing the enemy to attack her with a normal attack, miss and then she'll hit back with her OP strength... I guess? me a noob http://xn--hckqz0e9cygq471ahu9b.xn--...83%AA%E3%83%88

but she'll come much later, so preparing a dark team for now, best just to get the main ones..