omega is only 50% power level shenanigans, and 50% "maximized potential of x-gene" That is a bit of a vague statement. The "max potential" means basically, no Iceman can be stronger then him, his kids won't be stronger then him. As opposed to Scott+Jean and Sinister trying to get their genes to make super powerful mutants. scotts powers, could in fact be stronger.
Also, even on raw power level, Iceman is pretty retarded at this point. He is immortal, ice powers work on a global and beyond scale, he can make clones, he can turn into water, or ice.
Originally he was just covered in ice, could make ice form.
now he turns into ice entirely. Can be broken, melted, etc, and reform himself. Make clones as well.
He can create absolute zero. Can effect weather on a global scale. and so on.
As noted, hes kindof a bro. and doesn't normally tap into that stuff. Particularly as a kid, didn't know his full potential is the retcon. He got injured in ice form, and was able to heal himself. working with Emma.
Alt universe versions of him, have gone full out. 616 main universe is still human.. but does have full access to that level of powers if something sets him off.
Who is and itsn't often varies by author.
Xavier and Magneto are not confirmed Omegas.... yet.... We have Magento controling the Earths eletromagnetic field, or "sensing" phoenix and metal objects outside the solar system.
Elixir for example is also an Omega level mutant.
However, his powers are not typical for "power levels". he can control organic matter. Cure anything. no known limits.