If can't move belt with your hand at variator end its pinched. You could start it to release belt then re tighten just quick on and off .If nut not tight nut will come loose and outer pulley can hit the case and do damage .

gave it a test run on its center stand, seemed to work fine, only thing that I noticed (not sure if its its normal operating behaviour?) was that the belt vibrated/wobbled up n down quite alot, top and bottom between front n rear pulley mainly on deceleration I think..... is that normal?

Probably less when under load .

Very likely normal, I was surprised how much the Burgman one I changed videreated but it gave no problems over many thousands of miles.
I have notices that since a new belt was fitted to my Xmax the slight whistle it had on over run has gone.
May be worth giving the clutch and variator bolts one last check before buttening it up.

Piaggios have a plastic roller wheel in between variator and rear pulley presume that's to limit movement of belt .