You're testing my memory now Charlie, only done this engine once lol.
We didn't slacken the camshaft pulley I don't think from memory and had no issues. I think it will be because you lock the rear cam, and cause of the design that locks both cams, so they will be loosening the cam pulley so it doesnt put any stress on the gears inside that connect the cam together while tensioning and also so the belt is evenly tensioned so nothing turns slightly when the locking tools are removed. Makes sense now I have read that tbh. I assume its like a 16v Fire engine and its literally just the bolt that holds the pulley and there is no woodruff key, which means that pulley can spin independently while you tension the belt up.
What i did was removed the aux belt and then the pulley from the crank with my impact so the engine didnt spin, no need to lock the flywheel, be gentle obviously with that, they arent that tight so don't need to go full welly on the impact. Alternatively you could put it in gear and get someone to hold the foot brake and that will stop the engine spinning while you undo them 3 bolts.
Then removed all the required parts to get access to the belt and cam lock, which from memory is the airbox (at least on the alfa mito i was working on), top belt cover, whole engine mount (jack under the sump), loosen that metal pipe that runs over the top of the engine that side, think that was it.
screw in the spring loaded cam locker, turn engine from crank until the locker clicks in.
Undo the tensioner and disgard
take belt off
change the waterpump while there
then fit in reverse basically, the belt should have 3 lines on and there is a mark on each pulley - its not essential you put it on so these line up but will be a good visual guide in the future so worth the effort. The high pressure fuel pump at the rear can be spun whereas the cam and crank cant for obvious reasons. I went from crank, up round the idler then cam then back to the fuel pump which i made sure was in the right place for the line on belt to match up.
May as well replace the aux belt and tensioner while there.
So slackening the cam pulley is, imo, not entirely necessary but i can see why they say to do it, but really there is not much tension there.. nothing moved and the car drove 200 miles home the next day no issues. :shrug: