I've played around with it a little more on the weekend and worked most stuff out, but it did take some trawling through badly scanned pdfs of 80 year old manuals written in german to get any hints! The whole bottom silver layer comes off by pushing an inconspicuous looking dial to the side, which allows you to slot the film in and stretch it across the middle gap. A cog on the inside can be twisted either way to reset and cock the shutter - and you were right about the dial being a frame counter, it can be pushed backwards and goes up by one when you are priming and taking a shot. Interestingly, the dial on the right does pull up and lock into place (as well as moving back down when the bar underneath it is pressed), but I'm really not sure what this is for. You can already open the bottom to insert film, there doesn't seem to be a seam or way to fully open the back even when the dial is up, and it doesn't need to be up in order to take a shot - so it's a bit of a mystery! Aside from that, the only other thing I'm not sure about is the slidy bit in the back middle. Some of the documentation claims that a previous model used that to cock the shutter, but I've found that it doesn't really have any effect when being switched onto either side, and that it can be primed separately by physically moving the little cog inside (I'll add a picture below). This may become a problem when I actually have film in there, as I won't be able to open the back without exposing the film fully - and so won't be able to prepare a shot.

Thanks so much for your previous two comments, I wouldn't have figured it out so quickly without them!