I am not counting Alola Sandslash out, as I did make note of its incredible offensive potential, as you yourself mentioned. But outside of trick room, which it probably won't get, it won't have much of a niche in battle, especially on the defensive side.
I never said Alola Ninetails was Ice/Fire, I was comparing its ice version to its fire version, looking at both offensively and defensively. Fairy is one of the best defensive typings, you don't have to tell me that. But adding a defensively weak typing to that combo, severely brings down the defensive qualities of the Fairy type. Coming in weak to Stealth Rock, and being doubly weak to Steel (moves such as Bullet punch and Iron Head which are becoming more common), as well as fire, rock, and poison greatly diminish the defensive potential of Ice/Fairy Ninetails. It has some good resistances in the form of Bug (U-turn), Dark (Knock Off), and Ice (Ice Beam), I will give it that. It has good potential as a sweeper, so if they go that route it will def form itself a niche as the ultimate Dragon killer, but if they keep its stats roughly similar to its Fire counterpart, aka decently high speed, low special attack and decent bulk, it won't really make many waves. Not to mention as of now it has a bad ability. So right now, without looking at stats, I think its Fire type counterpart is superior, especially with Drought.
Weavile functions as a great sweeper because it has two great offensive types, Dark and Ice. Both are more frail types that flourish as a sweeper. That is why it is effective. Electric/Psychic, not so much. Both aren't exactly steller sweepers, though the electric typing is decent. The only moves that Weavile certainly has to fear are Vacuum Wave and Mach Punch, which aren't exactly common. A-Raichu has to fear Sucker Punch and Shadow Sneak which are certainly more common. Not to mention Knock Off will destroy this thing. You are right in that it has potential as a sweeper and an added STAB is great, the psychic typing isn't a good typing to have tacked on.
As far as Raticate goes, I didn't mean A-Raticate had bad abilities, I said that its abilities didn't give it anything over its counterpart, who has the same ability in the form of hustle, and a better ability in the form of Guts. But I do agree that Dark/Normal is better than pure Normal, even if Fighting moves destroy it, because they destroyed before as well. Hopefully, though I'm kind of doubting it, it keeps its high speed stat.
I agree with you on the A-Marowak points. 3 immunities is grand, and while it comes with some bad weaknesses, crippling physical sweepers, a potential priority move itself (Shadow Sneak), access to Thick Club (hopefully), and immunity to burn will certainly help create a decent physical attacker (if it gets Flame Charge too it might have potential to outspeed as well). I was looking only at it defensively.