OK, based on what you have said, don't beat up on yourself.
The first thing to say is that suspension of an individual whilst an investigation is undertaken is pretty standard HR policy. It doesn't mean that you've done anything wrong, it just means the process was found by HO to be sufficiently broken to warrant an investigation. You should be paid normally whilst you are suspended.
If it is shown that you have followed most of correctly, that is good. It is just a relatively minor error in missing 2 days (in my opinion).
I don't get the bit about the keyholder (who presumably works for you) not being authorised. I assume this could mean one of two things:
This person has never been authorised (by HO?) to be a keyholder - but you didn't know it
By failing to sign correctly, the keyholder's authority is automatically nulled
It is your job, of course, as manager of the keyholder, to undertake audits to check they are doing their job correctly - you seem to be able to demonstrate that most of the time it has been done correctly.
I suggest that you spend time preparing your case, be prepared to admit the error but point out the (many?) good things you have been doing in the job.
My guess is that you will asked to attend a disciplinary interview to hear your side of events.
You should know the disciplinary procedure of your oraganisation (as I assume you use it from time-to-time with lax shopworkers), so you know the process.
From an HR point of view, this doesn't sound like a case for gross misconduct or dismissal with notice. It sounds like it could end up being a written warning of some type - which I'm sure you will know means don't do it again.
No-one died, nothing was stolen from your company and your actions didn't result in any other loss from the store.
Good luck with it.