Have to remove door closer as fitted to the wrong side of the fire door and refit to correct side this leaves 6 screw holes on the other side of the door how does this effect the fire integrity of the door? Can they be filled and the door repainted or polished?

Yes. If they are short fixing holes they can be filled with acrylic wood filler then sanded and painted over.  Make sure the self-closer is suitable for use on that side of the door leaf, some closers aren't suitable for the push side.

Can I screw (4 corner screws) a full length mirror (45cms x 120cms) into the fire door or must I use adhesive pads?

My advice would be to not fit a full length mirror to a wooden door due to the weight of the mirror and its effect on the hinges and fixings as well as the shape (tendency to bow/twist) of the door over-time.

In terms of it being a fire door, clearly where the weight of the mirror does have such an effect this could mean that in a fire the door could lose its stability (eg. warp, twist or bow) and cause the door to fail prematurely.

Have a fire door that someone has previously tried to fix. Now the door where the hinges screw in are completely stripped out. the door on the back side is also cracked. Is there any way to fix this door. larger screws were already placed in the holes so that method will not work. any advice would be great. 

From your description regarding the hinge fixing points AND the door being cracked, it may be better to replace the door.  Without seeing the full extent of the defects its impossible to be more specific.

I had a composite fd30s fire door fitted with overhead closure put on the outside door which it should have been put on the inside of the door now I have 4 screw holes in the brand new door am I in my right to ask for a new door to be fitted 

So the installer seems to have made a mistake? The installer should be contacted and remedial works, approved by the door manufacturer, should be carried out. Subject to reasonable acceptance by the client.