A fiberglass pool is far superior to a vinyl liner pool. It is also about twice as expensive.
Pool with a vinyl liner.
Big NO NO ! It will soften or destroy the vinyl
When you say a "pool liner" I am assuming you mean a vinyl liner as normally called a liner for short. If so then no, the liner cannot and should not be painted.
As long as the vinyl liner pool is inground, it can be converted. But you'll probably have to remove all of the vinyl pool elements until you have reduced the pool to a big hole in the ground and then start from there. All it takes is time and money!
High concentrations of chlorine can and will destroy a vinyl liner.
Yes there is no problem with using salt in a vinyl lined pool.