so what should i do with this piece? i want to make it match my dark bedroom furniture but my painter is telling me that the veneer is too thin... that, once he sands off the glossy coating, there's no good wood left to paint or stain.
Unfortunately, he's right. Your built-in is veneer, and the process of removing the poly seal will ruin the surface underneath. You therefore have three choices: 1. paint it 2. replace it 3. leave it.
We would leave it for now and do everything else. The built-in is a beautiful piece and, while we don't know how dark your furniture is, it might not clash too much. The bigger problem is your floor. That should definitely be stained dark if you want any dark wood in your bedroom at all. We would leave the built-in, stain the floor and work with off whites on your white walls to bring everything together.