OP, I think you need to think about setting much clearer boundaries with people, both at work and at home, in terms of what you will and won't accept. All of your examples seem to involve you bending over backwards to accommodate other people's needs, and then getting hurt because they don't behave the same way towards you.
For example, the employee who "insisted" on taking annual leave at the wrong time for the business. It doesn't work like that. Standard employment contracts have in the ts and cs that annual leave can only be taken at a time agreed with management. If an employee asks for leave at a time that really doesn't work, say, sorry, no. If she shouts and screams, say "I'm sorry this is upsetting for you, but it can't be helped" and leave her to calm down and get on with it. If she threatens to take the time off anyway, remind her that that would be unauthorised leave and if she does she will be disciplined and ultimately dismissed. You don't have to be nasty about it, just calm, reasonable and above all, consistent. People might moan a bit, but eventually they will realise that your behaviour is always fair and they will respect that. And if they don't, allow them to walk away- they aren't any great loss to your business or your life.
Where the employees' mothers are concerned, just don't deal with them. Politely explain that it is a confidential matter between you and the member of staff and you are not able to discuss it with anyone else, and end the conversation.
If the one who has just left you in the lurch is not working her contractual notice period, you don't have to pay her for the "holidays" she is taking. You ultimately can't force her to turn in, and I agree that suing probably isn't a realistic option, but at least cut her money off.
You seem like a nice person who is trying really, really hard to be kind. You say that when you are assertive you are accused of being unreasonable- well that's not because you are unreasonable, it's because these are selfish, unprincipled people who will fight their own corner and say whatever they can think of to win. Have some faith in your own judgement and stick to your guns.