How did you manage to set the light/proximity sensor in its right place? I've reopened my phone 3 times, but it's still not working. Everything else, including auto-brightness works fine; it just doesn't shuts off the screen and activate the front speaker when in calls and phone near my ear :/

Maybe some dust is inside the connector and prevents it from correctly fitting. Or you may have bent the cable too much and now it is damaged. Or, as you say, the replacement was broken from the start :/

Where do you apply force? Where the camera is or more towards the upper right side of the phone? In the first case, you may have a defective sensor cable, maybe the camera is not correctly soldered on the strip wire; in the second case, the cable sensor may not be correctly plugged into its connect for on the main board, even though the metal plate covering the 4 connectors from the front panel should apply enough pressure to keep it in. There may be some dust inside the plug that prevents the connector from completely fitting inside.

You can change the camera, but your iPhone 6 will get no benefit from that, since it's built up to use the original camera.