It's very easy to handle situations badly in the early teenage years, when your DC are giving you new things to deal with every day! 
But you really do need to do things differently next time, clutter, or I think you are going to have a lot of problems...
Firstly, your reactions and behaviour were unfair. Teenagers' plans do fall through. Your DD texted you to let you know; she asked for what she wanted (coming home later than usual, at 10pm, as a sort of 'consolation prize' for not getting the sleepover she'd wanted); she had a bit of an argument about times (like they do); she agreed to compromise (and come home at 9pm) and then she stuck to it.
So in a nutshell, she communicated with you fully and left her friends/social gathering/party/FUN and came home early - even though she'd been hoping to have a sleepover and must have been disappointed - not because she wanted to, but because you wanted it and told her to.
This is really good - your DD behaved very well!
When she got home, she could reasonably have expected you to say "Hey, hi! :) Thanks for sticking to our agreement. You must be really disappointed you didn't get your sleepover. And I appreciate that you came home earlier than you wanted to. Thanks!"
But instead, you gave her a hard time, sent her to her room, carried the argument on by following her upstairs, took her phone, grounded her and then slapped her. :(
If you are not very careful, and if you do not recognise and appreciate when your DD communicates and compromises with you, she will stop doing it. There are other, much worse ways your DD could have handled the situation last night, and she might choose one of these next time. She could have ...
- Told you about the cancelled sleepover, but just refused to come home early, and come back at the time she wanted.
- Agreed to come home at 9pm, but actually not turned up until the time she wanted... Or even later, to 'get back at you' for being unfair.
- Not bothered to text you to tell you the sleepover was cancelled until later - 'til after 9pm, or after 10pm, or until whenever the friend's dad actually kicked them out (this happened with/to my DS once at midnight, after the last bus had gone)...
- Not bothered to tell you the sleepover was cancelled at all, and gone somewhere else - perhaps just to another friend's house, or perhaps going with another friend to see if they could get in to a pub or club... And you may never have known where she was...
The alcohol issue is a separate one IMO. You never get anywhere discussing alcohol/drugs with teenagers who are actually still drunk/stoned/otherwise intoxicated - you need to leave any/all discussion until they are sober again.
If you really feel you must, the only sensible thing to say when they come home drunk/stoned is "Hmm, well thanks for sticking to our agreement, but I'm not happy you've been drinking... I'll talk to you about that in the morning. Good night".
Usually, the best time for these conversations is the next day, but you lost your chance on this occasion... I think you probably need to leave it until the next time she asks to go out somewhere, and say "Yes, sure - but if you come back drunk like last time, you will be grounded and you won't be going anywhere else for a while".
Oh and teenagers love drama... Your mum definitely gave very good advice when she said "she'll probably come back home at tea-time and I shoudn't panic and just act very blase' when she returns".
It's a bit ironic that most of us mums don't get wise until after our own kids have stopped being difficult!