KHAAAAAAAAANNNNNNNN! Sorry...
I don't know if I follow. The front end doesn't handle what games are included in mame (its just a tool for launching roms). To see what games Mame supports (for example) check the documentation or mame.net.
Secondly, roms may not be included in mame for numerous reasons (not working, too new, etc.). Once again, mame.net may be of some help.
Third, I don't know what you mean by being the 'legal owner' of a rom since most roms are not legal - unless you're technically the copyright holder.
Hope that helps.