Yes u can u need the battery, a fully working board, the housing, screen and all the other bits screws etc if u find that feasible to do then yes u absolutely can :)

The Audio issues on the 7 and 7 plus are usually caused by flexing/bending it's a common and well documented problem and decent places will be able to fix that for u for a decent price. Fixing this will probably be cheaper than replacing the board... replacing the board u may not have audio disease to begin with but if ur housing is bent or twisted in any way then u may just cause the same issue on the replacement board. The solution to audio disease is to reinforce the pad that comes loose with a wire that gives it a little flexibility and u shouldnt experience the same issue on that board again, so the repair is quite a solid solution.

(1) Have a look if the original can be fixed (2) Have apple replace the front screen and replace and programme a new button for u (3) Have a look at the really not great bluetooth home buttons that click and use bluetooth to connect to the phone eg JC home button

Dont know what ur talking about removing the front panel??? U need to remove the front screen to do any repair in any iPhone since the iphone 4s otherwise u cant get in. I would personally change the battery see if it works and starts up ok, then just plug in the charging flex and see if it works ok check microphone etc with it just plugged in then fit it fully, after that test again and then fit the earpiece.

I dont know what u did to cause that so not sure if I should recommend a repair, however first thing to try would be a new screen, not sure if u still have the home button and the top camera flex seems torn as well. iPhone 7 のスクリーンの交換

Heres the screen Moto G6 Screen - Genuine And the guide for replacing it Motorola Moto G6 スクリーンの交換

The board itself holds all the data well a specific chip on there holds it… it's not so simple to just get just the data element over to another phone, it's more practical to fix the board… In order to transfer the data would require a lot of skill havent really worked too much on the xr but I'm assuming it's similar to other iphones, u would need to transfer the eeprom, nand, baseband and cpu to another board and possibly others like wifi if u cant reprogramme the nand. That for data recovery isn't very practical ur better off saying what's wrong with the phone exactly for better advice

Nope nothing is ever interchangeable apart from maybe the 2 bottom screws…

It should just plug in check it's been folded like the original and it should just plug in compare it to the original battery just in case u have the wrong battery for the phone...

Really ifixit is for people to fix their own stuff so have a look if there is a guide available for ur item and take them apart and see what u can do...