Probably too late for the OP, but in case anyone is having the same issue, here is how I removed the rear bench seat to get to the seat belt casing which one of my children had jammed a small toy into
First, follow the instructions in this video to remove the upright section of the back seat:
I emptied my boot and put the double seat in that space, the single seat lifts out completely so I put this inside to give myself some room.
For the next bit it's easier if you have some room so move the front 2 seats as far forward as you can.
First, you have to lift the bench seat upwards by grabbing the front where it meets the car lining. Be careful not to yank it upwards too quickly as there are 2 pressure sensors connected via wires which go somewhere under the car and connect into the onboard computer to set off an alarm if someone is sitting on a rear seat but hasn't buckled their belt.
I found a video of someone removing a rear bench seat from a MG5 and had an inkling that it would be similar in the ZS. If you want to see the video you can watch it here (the bit that shows the clips is around 20s in):
There are 2 retaining clips one on either side so I worked one side then the other to make sure I didn't pull up too much and compromise the pressure sensor.
Once it has come loose at the front then you'll need to free it from 2 hooks (not sure what the technical term is sorry) located near the back of the bench. To do this you have to work it back (towards the boot) and then up. I've included a couple of photos below to show you the mechanism. There are 2 of these so you need to do both to free the bench. Bear in mind that the bench can't be removed completely as the pressure sensor wires connect it to the car. I was able to do what I needed to with the belt buckle with the seat bench just being free from the retaining clips/fixings.
If anyone has a similar issue to mine and needs to remove something stuck in the seat belt mechanism, I ended up breaking the outer casing off as I gave up after 2 hours with tweezers and other tools to get the small plastic toy out. So long as you dont damage it too much it was easy to glue back together in place around the actual belt mechanism. There were some second hand belt mechanisms available on ebay that I looked at buying but you need a large torx bit (I estimate either a t60 or t70) and then you'll need to unclip the seat belt sensor cable from where it's cable tied in.
All I can say is there will be no more small toys in the car....