Is there a difference between imidazolidinone and imidazolidone?
According to the NIST ChemBook, 2-imidazolidinone and 2-imidazolidone are synonymous. The official IUPAC name is imidazolidin-2-one. 1,3-ethyleneurea is also another name for this same molecule:

    



But, as you note, tiny changes in spelling can sometimes be much more tricky than this. To take an example from this family: imidazolidinone and imidazolinone (note that the later ends in –inone, not –idone) are ketones formed from imidazolidine and imidazoline, respectively. Imidazolidine and imidazoline are partly saturated and fully saturated imidazole derivatives:

   

Imidazolidine is a proper IUPAC name, but the official name of imidazoline is 4,5-dihydro-1⁠H-imidazole.