The people who are away on honeymoon will almost certainly have told their solicitor and/or agent or how would anyone know its their honeymoon and that they don't want to move until they return. Its entirely possible that that information has been withheld so that when the bombshell is dropped its a "fait accompli" i.e. its not worth anyone trying to do anything else.
Unfortunately you have no control over what others in the chain do, there are always gits that must have it all their own way, the best you can do is be very matter of fact and try to be as flexible as you can. Try not to stress too much about it, remember that there is a major difference between delays and problems.
I would expect you are right about the person not willing to wait after all if they find a different house or buyer then it will take much longer, if they decide to stay put they will have spent out on legals for nothing.
All being well, by Christmas this will all be history and this extra few weeks will pale into insignificance in the grand scheme of things.