Is it possible to allow a merge between 2 coalitions if both agree. We have tried but other coa says they cannot resign as hege because no one left? Is this correct? If so why not permit full mergers?

Unfortunately, I didn't get your concern quite straight, could you please specify it in more detail? What is the issue with a hegemon? 

Sorry, we had a single player try and leave his self created coalition and join us. However he said he was unable to leave unless he had someone to take over from him saying it was not possible. is this a game restriction or just his own issue?

its a game restriction, the alternative is for him to start another account. make that the Heg and then he will be able to leave with his main account, just about the only time I would actually condone having an alt :)