Try this. when lifting the mylar try pulling it at a full 180 degrees whilst heating it .protect the art work where you have removed the mylar all ready with a thin piece of cardboard to protect the art work from heat.keep moving the cardboard as you go and use more more heat the hotter the better. If you loose art work SHIT HAPPENS you CAN touch up !
Now the art work trying to write letters or staight lines free hand looks crap (sticks out like dogs balls ). The way to make it look professional I use model makers painting tape you get it at a hobbie shop, its yellow in colour and it's great. All you pin guys out there you must buy it ! It comes in different widths 4mm up to 30 mm which is great for dead straight lines. Its also very thin like cig paper and  low tack great for back glasses. If you get the 4mm tape use it on the bottom and the front edge of you letter that is damaged , the E for eg  ( by the way the tape will arc very easy ) such in the case where you letters are arced. Use a very small art brush, I also use the little paint tins that you can buy at the hobbie shop when your there. Buy a range of colours there cheap and they have heaps of different colours and there turps based if you make a mistake wipe it off with a rag and turps.
So come'on take all the mylar off and touch up the whole playfield. When your finished I would re-mylar the playfield. It will look brand new when your finished.
Ive done heaps of playfields and back glasses If you need help Im here