The front threshold is resting on existing tile and the back of the pan is resting on loose tile to "level" it. The installer insists that the manufacturer recommended against setting the pan in quick set or sand.
The drain is a brass compression drain and if you shine a flashlight in the drain you can see movement when my husband walks around in the shower. There is not a lot of flexing, however there is movement. He cannot feel the flexing, but it can be seen in the drain. My question for you is; should I be concerned about the pan leaking in the near future because of the flexing and or drain movement? Thank you so much for your help,
Acrylic and other polymer shower pans--as opposed to a tile constructed completely of tile and mud--will always feel hollow, no matter how good they are. High-quality, heavy shower pans will take you closer to that feeling of solidity, but I have yet to find one that feels as solid as a mud/tile pan.
As for the feeling that the shower pan is flexing, that's a different story. If it's a cheap shower pan, you might feel some flexing within the pan area. If the whole thing pan--end to end--feels like it's flexing, then it might be bridging those two areas of tile that you mention, without adequate support (such as shims) below.
It might be supported just at those front and back points. So, bridging would be a bad thing.
There should be no gap below the shower pan. It should be fully supported at all contact points.