it looks like you have handed over your device to a scammer who is getting dubious knowledge from YouTube. It's sad, unfortunately quite common. Rule one: never ever repair your iPad in an unconfirmed repair shop. This fragile device is easy to break in the wrong hands. Requires knowledge and years of experience in repairing microelectronics. But, to the point:
Touch problems in the ipad 4,5,6,7 series, where the LCD matrix is separated from the digitizer, are related to the extremely low quality of the Chinese, polycarbonate interchangeable glasses with a glued digitizer. (I will not mention the electronic reasons at this stage - which are also very important). Replacing the touch screen in this series, only 4-5 of the 10 screens are usable.
I wonder why the button is disconnected and the touch function is lost. Correlation known from iPhones. Did your Home button also get damaged when the original screen was destroyed? The Home button is connected with a different ribbon and all signals are separated. If the button does not work, the tape itself is probably damaged or the port on the motherboard is not properly tightened. And finally the connectors cover: their absence and the screen screws you write about are evidence of your mechanic's sloppy and rush. That's a bad thing for a microelectronics technician. Last question: did your iPad work when you gave it in for screen replacement?
Boot loop problems usually arise from problems with the motherboard. Untrained hands make microcracks when removing (unglue) the motherboard from a housing. Commissioning after updating requires automatic initialization of the installed components - if this does not happen, the system waits via pre-programmed time unit. If this does not happen, the system says "time-out" and reboots automatically. This could go on forever. If it concerns non-essential accessories (such as compass, flashlight or Home button) the system will not register the component in the new build and it goes ahead. On the other hand, if the initialization hits a damaged component causing a short to ground - the system is unconditionally restarted. Another important reason is the problem with allocating a new system in memory or updating user data. In this case, a restore with the factory settings will suffice.
there are service methods to do it without a button, but it is not a home method by an untrained person. The first is the service pinout of the control pins on the motherboard. Closing the circuit places the processor in a service state. The second method is to use the button's port and close its circumference manually. As a result, we can get to the service modes (there are three: the first, second and third level). I do not recommend this for fun at home.
I advise you to give your iPad to the right hands. I know two good studios in the US: NorthridgeFix, CA (NorthridgeFix.com & IpadRehab, NY (ipadrehab.com) or in the EU: studio.N / east.EU (studionapraw.pl) & RESQ / west.EU (resq-repair.com). You will certainly find help there.