There is this tiny metal thing on the back of the flex backplate connector. Sometimes is sticks on the back of the original display instead of the connector and then touch id will fail to work. The part i mean is marked on the image.
Yes you can put the iPhone 8 + digitizer on a 7+ iPhone. And it will work just fine, only the display. You need to use the orignal homebutton and sensor/frontcam cable. The screws in the bottom wont fit unfortuanately
I've done one yesterday, it's the same way as a tab 3 & 4. Remove the back cover. (difficult because of the hard clips) Disconnect the top digitizer cable. Heat the front around the edges, and use a suctioncup with guitarpicks. Lift of the screen, clean the whole %#*@ thing an stick a new digitizer to the frame. Connect the digitizer cable. Click the backcover back in place. ( I had to use a ridicoulus amound of force to make it click) The digitizer and lcd are not fused together. It was a model in Holland.
I use a 2 component expoxy glue and a injection needle to apply the mixture very precise. Apply the glue on the frame and place the frame on the glass, apply pressure for 2/3 hours and it wont come off ever again.