Is paint lost avoidable?

You would want to use UV safe type glass (museum quality or whatever). Regular glass won't work long term. The material for the mat also is important (acid free and all that).

And its not just sunlight you have to worry about, flourescent lights also damage cels over time (messes up the lines on the front of the cel or something).

But the lines are UNDERNEATH the cel...?

When people frame cels on mount board, do they stick it on mount?

Im planning to just frame with a cel bag over it.

There should be lines on the front/top of the cel and paint on the back (at least there is on all the cels I own). They take the douga and copy it onto the blank cel and the lines end up on the front, not the back. The paint goes on the back. That way the lines are sharper and more crisp.

Hold the cel up at an angle and you should be able to see the lines on the front.