First I'd say get what emergency pay you're entitled to now and worry about the face to face later. Most people aren't benefits experts or lawyers so many people will make genuine mistakes on their forms, it's correctable. So I'd not panic about prosecution.
I don't imagine UC even have enough agents to tackle the tsunami of 5 million physical interviews. That's like 4.5 x Birminghams. My feeling is it might take months to skim the top if they do it the 'old way'.
Yes I recently experienced everything you mentioned. I had the interviews and demands for invoices several times.
The way they couch the questions can be confusing and get you stuck in the wrong category and denied benefit.
The whole point of going on benefit is that you can't support yourself isn't it?
So you're not gainfully employed are you?
I was pulling my hair out with UC demands-till I sat down with a Citizen's Advice lawyer for a couple hours. (You can ring also)
I suggest getting solid legal advice from Citizens Advice before you step into a UC building.
I did and Citizens guided me beautifully. I'm very satisfied.
(I also do my own accounts.I'm registered as something in health and well being field.)
Don't fret too much as the whole UC system is changing daily and who knows, it might be completely different in a few weeks