Hmm..My post disappeared...reposting from browser cache...
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The firewall should be slanted...

Its probably slanted slightly right and slightly down. This helps to compensate for the torque of the motor and deliver right/down thrust. By placing the engine that way, you won't have to artificially have your rudder control surface with a constant left turn in it and your elevator with a constant down in it. The result will be more equal control surface throw.

The more slant, the more the designers probably figured your going to overpower the plane.

As far as slightly cracked firewall formers, some questionable sanding, some slightly loose covering...I've never seen an ARF that didn't have at least one of those issues. Hell, even when I build a model its going to have at least one of those imperfections. And if it doesn't, it will after the first trip to the field...if nothing else but minor hanger damage. Now, if the covering has puncture marks...I would say that goes into the unacceptable range.

For all what you said, I would rate this ARF a "C"....there are worse, there are better....

I would probably just epoxy the firewall cracks if they weren't too bad and there was only one....but you didn't include pictures of that.