The roof hatch is there only because at some point in this tank's history it was used for carrying stores, and these obstructed the driver and commander's access and egress. Mark IIs were not built with roof hatches.
The covers for the periscope holes are simply missing. I'm not sure what you mean by the "observers hatch" - do you mean the visors in the cab front? If so these are the same but there are detail differences in the cab front between the Mark II and Mark IV that you will need to take into account. These are explained in various books.
Yes, the hatch more back, the drivers hatch is already fixed.
Thank you for yor informations and looking for me in the old forum threads. But if I click those links there's somethin to download I don't know? Can you maybe tell me what it is? I don't like to download strange things at all..but I thinks it's ok..