I'll number the points to make this a bit easier.
1.  Sure thing, we can change it to that other reason.  It would probably make sense that after dealing with so many magically empowered enemies the knights might decide they need a mage's assistance.  Up to you guys though, I was just thinking of this from a different way.

2.  All cool, the mana glow really makes no difference in regards to this.

3.  I guess my wording was a bit weird with this one.  For most mages, gravity magic is quite dangerous, because you are susceptible to the effects of your own spell.  AOE gravity spells are incredibly effective for subjugating groups of enemies, but there is always a chance you get caught in it and end up hurting yourself.  Chrollo has a form of resistance to his own gravity spells, and thus can utilize gravity magic with more "broad strokes".  At least that is what I was thinking.

4.  As for his actual power I am somewhat unsure because I am not completely sure of the strength of mages in this world.  I am completely fine with you guys making that decision if you want to, but I would guess somewhere in the upper half of mages.  While he still is inexperienced and has plenty of room to grow, he is currently working right under the Headmaster of the college.  I would probably put him on the level of a college battlemage, but with less experience.  He has the advantage of wider use of gravity magic but lacks experience in combat, as well as proficiency in much outside attack magic.

Hope that did a good job answering some questions.  I am just struggling a bit to find the power scale for him because while there are a few minor mages in the knights there hasn't been much high-level magic shown.  On the other hand in Tyaethe's lesson recently someone stressed that the 2 things you should never fight head-on are a well-fed ancient vampire and an extremely powerful wizard.  I obviously don't want to be that strong, so I am trying to find some middle there.  Thx.