Could i Rotovate now before erecting a Polytunnel

I wouldn't. You still have to erect to tunnel + cover, therefore you'll be walking all over the beds etc.

If you've double dug in manure, if possible, could you put some boards down to walk on?

Then all you have to do is give the surface a wee forking over.

If you try and rotorvate once the cover's on, you may hit a stone / lose control and end up you and / or the rotorvator handles etc taking a flying lesson through the cover!