I know this route really well. No one wants to share a cabin; you book and hope hardly anyone else has booked and you get your own cabin by default. (Unlike restaurants where they try to bunch people together, the staff try to give berth travellers as much space as possible to maintain as much privacy as possible.)
However, sometimes all cabins are booked up and you do have to share (unless you've paid extra for the solo supplement or gone first class - both of which are very expensive jumps in ticket price).
The cabins are tiny. If you woke up in the middle of the night and someone was on top of you, you would struggle to fight them off due to the lack of space. If there are panic buttons, I've never spotted any - not sure anyone would think to look for one in that situation.
With the seated carriage, a guard checks on you regularly. It's not great for your back and you frequently wake up due to passengers getting on and off and not thinking to be quiet, but I don't feel vulnerable. I know there is CCTV and someone making a regular sweep. The Cally Sleeper staff are fab and they look out for everyone. Everyone in the public areas, that is.
Now that I know I could be sharing a berth with a man - note, not a genuine transwoman, but a man self-declaring as a woman to get into their same tiny semi-private room as me - I'll be sticking to the seated carriage, thank you.