A defective rear camera will cause the front to malfunction too, you need to open it up and replace that rear cam. iPad Air 2 Wi-Fiの背面カメラの交換 Follow @refectio guideline for re-assembly in his answer to this post: After doing my repairs (battery and screen) I have problems
In most cases, boot looping is an indicator of a defective battery, I would start by replacing the battery first. Replacement battery costs less than $20, make sure you buy from reputable vendors like iFixit, MobileSentrix, etc.
That “riser” thing is generic, you can transfer from any iPads. What you also need is a rubber gasket which doesn’t usually come with replacement home button iPad 5/6/Air 2/mini 4 Home Button Gasket You can get it cheaper elsewhere, or use salvaged ones with right cut-out
You can use heat gun to remove that sensor, but in order to solder onto the replacement screen, you need to use a soldering iron. Some companies sell replacement screen assembly with sleep/wake sensor pre-installed, see if you can return bought screen for a full refund and buy one with installed sensor from companies like MobileSentrix and InjuredGadgets.
In order to retain touch ID functionality, you need to salvage original home button from broken screen and transfer to the new one. Buy a replacement screen with daughterboard pre-installed (‘the middle flex’ circled in picture) to save yourself the hassle of soldering, from any reputable vendors like iFixit and MobileSentrix. Follow this guide for disassembly of device: iPad Pro 12.9" Display Assembly Replacement Follow @refectio guideline in his answer to this post for re-assembly: After doing my repairs (battery and screen) I have problems