To be honest, if you think CM Cress is the reason Cresselia should be suspected, then you definitely need to rethink your team building or something. CM Cress is so fucking passive before it sets up, and there's so many things that can take advantage of it before it sets up, such as Sub SD Cobalion, Doublade, Gator, Mega Steelix, Mega Aboma (if it's the non HP Fire variants), Durant, CM Melo, CM Reuniclus, Escav, Drapion, Mega Banette, etc etc.
The one set that makes Cresselia suspect worthy is its support set with T-wave. Holy fucking shit is that thing annoying. Not only does it never die in one hit, but its insane bulk lets it basically switch into any unboosted neutral hit. Although Cress is still weak and can't hit for shit, T-wave makes it a huge threat against offensive teams and significantly patches up its passiveness against most teams. All it has to do is spam this move throughout the match in order to cripple everything, then proceed to bring something in like CB Tyrantrum or SD Gator later on in the match to completely mop the floor with the opposing team. Lunar Dance is also an amazing move which allows you to play very aggressively early on in the match without having to worry about losing your win coin, seeing as how you can just heal them up later in the match, and because Cress is so damn bulky, it will have a LOT of opportunities to safely use the move.
Now personally I don't think Cress is broken at all. Even though its defensive set is very annoying it's still complete Taunt bait, and there's lot of Sub sweepers out there that can still set up all over it. The CM set is still decent, but like many have said, it's not nearly as useful over other good CMers in the tier because it lost its niche in beating Panda.
Gator is another story. Qwil is not a reliable answer to all Gator sets because Sub SD sets (which aren't uncommon) can set up all over it. Tangrowth has to be heavily invested in defense, or else it risks dying to a +1 LO Ice Punch after SR (which sucks because offensive Growth is by far its best set). Same shit applies to Amoonguss. Wrath is one of the only reliable counters to Gator, and while it does have uses outside of checking Gator, it's not the best mon in the world. Gator has plenty of checks before it sets up, but its Water-typing is great defensively and it has nice bulk, which allows it to take most non super effective hits pretty easily.
As stated above there is actually quite a lot of checks to Feraligatr, any scarfer that is faster as well as encore and other things.
I'm not entirely sure what you mean by "any Scarfer", because quite the contrary, most common Choice Scarf users fail to revenge kill Gator. Durant and Hitmonlee both fail to even come close to OHKOing it and a +1 Waterfall cleanly OHKOs them back Delphox can't OHKO it with Grass Knot. Scarf Tyrantrum can live a Waterfall, but fails to OHKO with Head Smash. That's unfortunate. So what are we left with?
Rotom-C. Yeah.....any scarfer is definitely a heavy overstatement lol. So all in all, the only reliable way to beat DD Gator is to kill it before it sets up, otherwise offensive teams are going to have one hell of a time trying to beat Gator, just like how bulkier teams really hate SD Gator (although offensive teams also dislike SD Aqua Jet Gator so there's that too).