Is it OK to set a standard horizontal window air conditioner unit on its side to fit into a casement window?

They make window a/c units for casement windows. They're just not as common as units for sash windows. Don't try to force the wrong unit into your window. Check the garage sales in an older section of town, read the for sale ads, check craigs list, etc.

Yes, they are right. Don't install it that way. The compressor vibration isolation mounts will fail; condensation will not drain properly and will make a mess; the liquid refrigerant will not flow properly and the system will shut down.

Most ACs have some sort of via so that water than condenses on the coils drips outside. Probably won't work on it's side.

actually not----a minimum of evaluate mounting the unit as close to the window as available then with the help of using tin flashing or different comparable supplies create a duct paintings to connect the two factors mutually (a/c and window beginning off)

I don't think that is a good idea. Gravity is involved with the process (I think). I know that with refridgerators, they say if it has been tipped, to wait a certain time before starting it up.