I sounded like you said the timing mark on the cam gear, when lined up with the timing mark under the clutch cover resulted in the the timing mark on the cam being in the 10 o-clock position. So you will have to take the cam gear off anyway.
I just replaced my cam chain and tensioner on Tuesday on my 1999 Foreman 450ES so it is still fresh in my mind. After installing the new Cam Chain and Tensioner there was no slack in my chain.
Maybe your cam chain tensioner lifter is bad or not adjusted properly, if that is even possible? I think it either works or it dosen't.
All's I know is that after lining up all 3 timing marks and re-assembling the topend, I hit the starter button and the thing immediatly came to life.
Other thoughts, I could see the timing mark (T) on the flywheel, but did not see any punch marks on the crankcase to line it up with. I gave it my best guess based on the stroke of the piston, then aligned the marks on the cam, cam gear and inner front clutch housing. I may have got lucky that I was not 180 degrees off.