An example would be the Soviet Union's Great Purge. Once you've started that focus, you must finish it till the end.

Not sure on this one unfortunately.

I've seen someone say you can add "cancelable = no" to the focus (this cannot be done through the site) - but I have never tried it, so cannot confirm.

If it does work though, let me know, and I will add it on to the focus tree tool so others can use it too.

I think you just have to write an effect and it will make it non cancelable.