You'll need to try another new screen. Sometimes the screens have a possibility of coming defected. Try connecting the old screen and see if the touchscreen works. Source: ( We own a repair store in Atlanta, GA called iRepairIT )
The magnets can also play a role in use of the power button. You'll sometimes find that the power button doesn't work because the smart cover is always active since no magnets are present. We have forgotten to put the magnets back for some customers at our store at iRepairIT (www.irepairit.com) but most don't seem to care about the smart case.
Also another good thing to use instead of Kapton tape is the adhesive strips when putting your iPad mini back together! Seems to work great if you don't have Kapton tape, or electrical tape readily available. We use it here at our store, [www.irepairit.com|iRepairIT], in Atlanta all the time! Works wonders 95% of the time.