Laughing with you, not at you as the saying goes, and I appreciate you're not laughing. Don't let it get you down.
I wouldn't like to second guess what Apple might say but I have always found them helpful myself.
If it comes to it, though I have not opened my iMac yet, removing the glass cover looks brilliantly straightforward as it is held in place with magnets. Take a look on Youtube for the mid 2010 model. Once you have watched a couple of people carefully removing the glass cover, it starts to look second nature. A couple of glass suckers and a bit of care not to twist the glass or drop the glass and it appears to lift off easily. I have seen some people do it over large mattresses and imagined they will unbalance if they stand on the bed to move around their iMacs. My preference would be to do it in a relatively dust-free and clean kitchen, on the floor. Having gently puffed the insect off the screen with a puff of compressed air, assuming it isn't actually in the screen itself(!), I would then carefully vacuum around the edges of the screen. I would hope that's then the end of your problem. If unable to buy a purpose-made cleaning rag, gently remove any finger prints with a clean lint-free piece of cotton. Preferably have a can of compressed air to hand, though at a pinch, if very careful not to hit anything with it, you could flap a clean baking tray or similar to displace dust that lands on the screen/glass panel; never blow. Then gently put back together. And I know just how patronizing that all sounds. Don't quote me on this because I don't know but I am not sure why removing the glass screen to clean it would void the warranty, unless you did damage during that process. Short of dropping the glass, which you are less likely to do if sitting on the floor, or somehow overbalancing the iMac, I am not sure how that would come about.
If worried, perhaps try phoning a Mac store in a different timezone?