when you buy an original iphone there is glue holding it together (the bezel and the housing). if you replaced the housing there is no more glue. try pressing 2 fingers, one on the camera and one directly on the other side the button should pop out enough to be able to push.
ok starting with the lcd screen right? u may have bought a faulty one check all the prongs on the motherboard same for cable number 1 and 2. make sure all the little tiny metal prongs are there if there is even one missing the screens could malfunction. then if thats not it then check the connectors on the cables. u said that the lock button doesnt work either that may be because cable number 5 ( the one that it plugs into is broken check that out. how did all these all these malfunctions happen??
u may need to get a new lcd screen or check the cables to see if it is jutting out even the slightest bit. when checking the cables make sure that all the tiny metal clips are there and none are missing from the rows.
your lcd maybe not fully on meaning u must plug it back in or another possibility is check the tiny metal clips on the motherboard and on the cable c if you see all the tiny metal rows if 1 is missing it causes the lcd to malfunction. the 2 screws at the bottom of the iphone have nothing to do with the phone they are only there to support the bottom to the top. also make sure that when u try to turn on the phone the screen is back in place or else it wont turn on. hope that helps