You are running around the wrong tree here. All this guesswork and "trying this, trying that"... oh well.
1.) All resetting NVRAM and SMC won't do anything.
2.) It should never be necessary to remove a CD/DVD by taking the Mac and eventually the optical drive apart. It's likely that you have damaged the drive or the connection in the process, if not anything else. The Apple Hardware Test would show if the optical drive is still okay. Better ways of removing "stuck" DVDs/CDs.
Needless to say that neither Apple's warranty nor AppleCare would cover this sort of "repair work".
3.) I'm missing information on which iMac model this is. No one seems to care?! If it's a newer one, it's unlikely to be able to boot either Leopard or Snow Leopard, no matter from what media.
4.) Try again booting by holding the Option key. Hold it all the time. There MUST be a screen coming up showing your available drives, at least your internal disk and the optical drive... unless you damaged all of it. Click on the arrow below the HDD then.
5.) Do you have a backup? Possibly a bootable one?
6.) Why do you, or did you, need to boot from optical media in the first place? Is there reason to believe that the Mountain Lion on the internal disk is not bootable anymore?
- If ML is "defect", you will need to boot into and use the Recovery System to first check and repair your internal disk, then re-install.
- If this doesn't work, you've screwed it all up, seemingly. You might need another Mac to download and create a bootable Mountain Lion "installer" on a USB drive, then making it a clean install on your iMac after wiping and re-formatting your HDD.