I missed this thread totally 
Diamond coat is just another word for clearcoat. It is supposed to be the most durable CC.
Most Bally/WMS games also have Mylar in high traffic areas - Like TAF has some around the scoops, Fishtales around the ball drop areas. This can be removed using several methods. The Hair Dryer method should be ok for mylar on Diamond Coat.
Just try a test spot first. Insert lettering lifting is a risk and there's no guarantee that it will work. MartyJ has done this and also applied some new insert decals to great effect.
I should have some spare decals left over (not all, but some) if you need them.
I'm still interested that they put mylar on it. I've read about the prototype playfields which I don't think had diamond coat.
From having done it a few times now
I can definately recommend building yourself a 'Nino' playfield stand. Removing the playfield will take only a few minues (label backboard plugs etc) and would be the best way to do it without question.
I bought a Bosch heat gun (fully adjustable) and shellite, a rag, spare time.
I should have a bit more to progress on mine after Sunday.
Good luck.
