Charles Helliwell's Answer:
You've inherited a right old 'Tweedledum' and 'Tweedledee' haven't you?
With regard to the headstrong eighteen year-old, do you remember just how headstrong you were at eighteen? You probably knew everything, wanted to change to World, had better ideas than anyone else and probably found taking advice from others a bit boring and tiresome.
Certainly, a logical, rational and businesslike approach is going to cut very little slack with your eighteen year-old. So, what you should do is to appeal to her sense of worth and sense of value, given what you know about makes eighteen year-olds tick.
If she thinks she knows better than you, give her that feeling of authority and responsibility. Make out a list or schedule of what is required in that shop each day and then tell her that it's up to her to figure out the way she wants to do it.
You hand it over, just like that and then take a step back. You tell her that it's up to her how she chooses to perform those tasks but you're not going to tell her how, unless or until she chooses to ask you.
Forcing her to take authority and responsibility will surely prompt one or two responses. Either she will take that responsibility and show you what she's capable of or she will default back to you and tell you that it's not her responsibility; in which case then, you have the perfect right to tell her what to do and how to do it.
If she fails to do what you tell her, then you can start the process of verbal warnings, written warnings and dismissal.
With regard to the fifty-something male, he's actually having a negative impact on your shop and the revenue potential of the business. In this instance, I would be speaking to the owner to advise them to let this employee go.
If he's still on probation you can follow employment law guidelines and terminate his employment under statutory law. If he's full-time you will have to follow the guidelines for verbal and written warnings, before proceeding to dismissal.
In the meantime, I would be inclined to set him some very rigid goals and targets and force him to choose between the anger and disappointment of his incredibly tolerant wife versus the stress and pressure of actually having to face up to working for a living. I think you'll find he'll vote with his feet more quickly than you will imagine.