01. Turn ON the iMac with the mouse button pressed (and kept press). This should eject any inserted disc.
02. Each new logic board installation should come with its own (new) warranty period. Consult your local Apple repair center, retailer, store, etc. for specifics.
03. No one has an idea of your location (country, city) - thus, quoting possible [layman or professionally speaking] legal actions is impossible. In some US states there are lemon laws - such as, have an item fixed - for the same problem so many times, and a replacement would be required; etc.
04. If I were you, I would try to boot from a
Disk Warrior disc. Maybe, just maybe - it could resolve the problem you have, should it not be the same problem requiring a (motherboard) logic board replacement.
05. To boot from the 'Hardware Test CD' - try pressing the <option> button, during bootup. Once all the bootable volumes are listed (and the arrow cursor eventually appears) click on the '-->' button.