I think there are several being in the warp like the personality of fire and water or minor"angel" that not a pawn of chaos.That's where you are mostly wrong.
For the "Spinning Mana Arrows" well that's easy as pie, considering the Warp, its just gonna have a different name "Spinning Warp Arrows". As for "Summon Elemental" if he finds a manual for it then why shouldn't he be able to learn it? Its not as if in The Gamer Manga there aren't different magic systems and they all work for the Gamer if he learns them, unless there are inherent conflicts between them.
EDIT: Of course everything is probably able to happen because of the Warp, but that's alright, the Warp is weird like that.
Yeah Warp arrow is doable in context of the warp.
I have some idea that The Big E operate on hard number of game logic that lead to him go on great crusade like civilization game.Well, that could lead to amusing awkwardness.
... especially when he turns out to be the first Gamer.
About the observe,Gm can nerf as much as Gm want,Observe provide the information that accurate without double meaning or misunderstanding on gamer part.
I think Gm can nerf on use mana in observe or high level will have more detail but that will become a lot like normal power and normal level up in this kind of game.
Like level up and plus point in our status,ofcourse the diffrent is we can min-max our status while normal warhammer game the GM will limite our grown rate in status.
Like
Str 5 toughness 5
In normal warhammer game str 5 is limit of human without enchancement or go chaos power up but gamer system can put all our point into str 10 without going to chaos,then we have normal human that can stomp Ork warboss with his barehand.
But that will be bastardize version of system not pure the gamer system or warhammer system.