Regarding the what if ,Jean had just re-trasformed after being lobotomized by the Sh'iar.Just like when Moira tests Jean in the Proteus saga and later in the Dark Phoenix saga,it is said here that the power grows geometrically (or exponentially) the more she uses it.When Jean re-transforms into Phoenix in UXM 137 after seeing Cyclops fall Xavier telepathically orders the x-men to attack her while she's still vulnerable,and they manage to hit her because she's stil weak.When in the What If Jean completely surrenders to Dark Phoenix she expands uncontrollably and consumes the Universe.
At this point Galactus (and everyone else) is a non-factor.
Giraud,just like Jean,had from the very first issue in which he was introduced psychic circuit breakers in place which held his power to manageable levels.And this was also the case for Rachel.When Necrom fought Rachel he said that he acceded only a tiny fraction of the Phoenix,but unlike her he wasnt afraid to use it.He destroyed a solar system,and the scale of his attacks was ever growing.While it is true that Jean is the only avatar,limitations are nearly always self-imposed.Necrom wasnt even a host,only a parasite,but used the power on a scale Rachel never dared to,because she acceded only the amount of power she felt comfortable with.
As for the Phoenix/Jean avatar thing:Jean dies on the Shuttle.In the moment of death,for an instant her telepathy becomes absolute,overstepping her mortal/physical limitations.She becomes one with all sentient life,past,present and future.Her telekinesis becomes absolute😖he becomes one with all matter and energy,time and space,all of creation.She can resurrect herself in a new body (hence Phoenix) and is reborn as an avatar of the force of creation,to which she offers an human viewpoint and the possibility to involve itself in creation from the inside.When she resurfaces from the waters in Uxm 101 Jean says deliriously: "the world,the stars,all of creation...are a part of me".However the experience was so overwhelming that she immediately began to negate it.
All manifestations of the phoenix that later bonded with Rachel,Madelyne,probably Giraud,etc. have been created by Jean in the moment of her transformation.
In X-Men the End Jean says to Kurt that the Phoenix is perfect for jumpstarting Creation,but some things require a human touch.Thats why she's unique,because she is Jean and Phoenix.The fact that she is human also means that she can be corrupted, manipulated, surrender to her dark side,etc.
In Morrison view Phoenix avatars are beings which are reborn through their version of the phoenix trasformation and can perform their tasks in the normal universe.Given that we'have seen various stray manifestations of the phoenix,and that even the survival of a Universe in NXM 154 did not seem much important to another Phoenix,it is seems that when the main task is accomplished,collateral damage or secondary consequences(even the destruction of single Universes) are not so important.We cannot say therefore that a phoenix host/avatar/wielder could not be permitted to reach enough power to defeat Galactus.
I put the Phoenix at the top of the hyerarchy because I like its symbology better.
The White Hot Room is the place where universes are born,are tended to when they are sick and die.It is an analog of the kabbalistic world of creation,where everything begins to exist.The Crown (the white light to wich the phoenixes fly at the end of Nxm 154) is described in the kabbalah as the alpha and omega of everything conceivable:not just actual universes,even imaginary ones.It is the state that trascends Creation,in which everything that was,is,will or could be exists in its archetypal form :"you were always there,waiting for yourself to arrive".Even Pre-Rec Beyonder was just the embodiment of a single Multiverse,while the IG derives from the Infinity Being,which to me was a previous incarnation of Eternity/Infinity.
Just like everything else they were conceived in the Crown and born in the White Hot Room.If the Phoenix really is an aspect of the Force of Creation,everything else descends from it.
The HOTU may be an artifact that someway directly taps into this force.We still dont know what its power source was.
So my hyerarchy is:
1) The Crown
2) Hotu
3) True Beyonders/Starbrand (multiversal level entities)/LT
4) Death/Oblivion
(Because even universes die eventually)
5) IG/Eternity/Infinity
and so on
The Phoenix as force of creation may be 1 or 1.5.The upper limit of an avatar/host is uncertain.
To Kgkg:if you're going by Pre-Rec Beyonder,then even him considered Rachel the only being in the Universe with the potential to be his equal (not LT ).And when Rachel made him experience the totality of life he realized he couldnt stop her even if he wanted,and felt humbled by what she made him see.