When it spins over do both the cams turn? And do the lobes line up correctly? 10-2 on no.4?
I had this scenario with my Zafira - had oil starvation to cam bearing causing cam siezure and stripped the lugs from the back of the cam gear.
If you don't remove the engine mounting and bracket you only make the job harder, and in any case, the mounting will stop the engine moving too far - that's what it's for
If you are replacing the sprocket, then it's got to be chains off, no?
Having done the chains on mine, I would say you should be able to do both chains and the water pump in a day, maybe a bit over if you have to drop the sump and clean it out, no need to take the engine out to do it. [I stand to be corrected by the Master (Carl).] :)