Stuck freewheel, Normal methods & tools won't work so I'm looking for a last resort.

Cheers, they tried a vice and where definitely turning the right way haha.
They also removed the Centre part of the hub and clamped that but still nothing.

Is there a way to take apart the freewheel and then cut it in similar way to removing a stuck seat post?

Nothing comes to mind sadly, you would be cutting towards the threaded bit of the hub or outwards from towards the threaded bit of the freewheel(whichever was being sacrificed).

It's one of those situations when you just need more leverage, longer poles and to be more firm with it. If you know a mechanic and have a freewheel tool with a socket fitting on it then you can use a long breaker bar and scaffold pole.

How To Use A Breaker And Cheater Bar On A Difficult Nut Or Bolt - YouTube

Like in that video but with the hub clamped in a vice instead of a car wheel. You will have a hard time to round off splines but try to be even with the pressure as if it does loosen and spin suddenly you could fall over lol