I know the disipline talk is imminent ...
I dont think you will be disciplined for going home on time, unless of course, the mess was caused by you, which I dont think is the case.
...and I think I'll go into it with the points I have raised, blame it mostly on bad management and ask for the argument not to be elevated.
If they have a word with you on the shop floor or in the office, then that is the correct thing to do.
If disciplined, make sure that you know exactly why you are being disclipined. As it stands YOU CANNOT BE DISCIPLINED FOR CLOCKING OFF ON TIME. Remember this. Of course, they might get you on some other technicality, eg. "2 weeks ago you came in late and the week before that, hence, we feel that we must give you a verbal warning." Its not gross misconduct so you cant be sacked in one hit (as some people seem to believe). If they call you into the office for an official meeting, ask to take someone in with you, who is on your side (an older member of staff perhaps or even the other guy who went home early - someone you know is in your corner); they should allow this. They will know you are young and will treat you as such, so having someone mature there might keep them in check. Remember, disciplinary procedures are a game. Never forget this. Its all one big game and the best players are those who do wrong in the course of their employment, but get away with it, while the bad players get hammered for doing very little.
Personally, I dont think they will mention what you did when you next go into work. However, for the next few weeks at least, watch your back. They might look very closely at what you do and look for any excuse to give you a warning. Lateness/timekeeping is a big favourite, so make sure you are in on time and dont get caught having an unofficial break.
You seem scared that you might get sacked. To get sacked from your job, in one hit, they would need a charge of gross misconduct. Typically, in retail, this usually means theft or anything that can be construed as theft. Physically fighting a customer or another member of staff is also a good way to get sacked. If you are currently on a final written warning (which I dont think you are), then all bets are off.
In the future, if you do get a warning for whatever reason, it will stay on your record for 12 months, after which it should dissapear, so its not the end of the world. The warning is supposed to be a means to make you see the error of your ways and force you to improve. It is not a means of punishment. So, dont fret.
For the record, I dont think any official action will be taken against you.