1. The drain connection is still critical even, or especially with, that liner. If the bottom plug does not have a good gasket, the water will leak through it. If the overflow is leaking, then the water will either get between the tub and the liner, or leak into the lower level.
2. Water between the liner and tub is not an ideal situation, but I would have to say that it is almost normal with that type of liner.
3. I would not say you made a bad decision, only that I would not have made it personally. It could be a bad installation or a mismatch between your tub and the liner.