unless the snapped disc damaged the drive, the shop is just trying to get you on a replacement. it is 100% possible to remove the disc drive and take the drive itself apart to remove the disc. I’d find another shop that is more helpful and willing to work with you on it, removing the disc and charging you a labor fee if the drive still works and if it doesn’t then offer you a replacement cost. you could also try it yourself, but there are dangers involved for an expensive machine like this. I believe there’s an ifixit guide on removing the disc drive however
as chris said back camera can cause front to not work as well. try disconnecting the back camera and see if the front camera works (may or may not). also, what repair did you do? I’m assuming a screen replacement (most common repair on a phone after all). Check to make sure the prox/front camera flex cable didn’t receive any damage as well. Clean connectors for both front and back cameras. Power cycle device (disconnect battery and hold power button for 10-15 seconds to flush all power out of the board). If nothing else you can try to do a factory restore via itunes and see if there is a software glitch that caused the issue. That’s about all I can think of that may help restore functionality though beyond replacing the (most likely just the) rear camera, which for some reason is more expensive on the iphone 7 plus than any other iphone I’ve seen including xs max.
you may be able to splice the existing cable to a replacement. however the replacement would most likely need to be an OEM one from another broken device or such, and would require microsoldering equipment/experience. a replacement home button may not have touch id but it’ll still work fine as a home button
check first dirt/debris in the connectors. if everything is clean and no damage it may be a board issue. test with the original screen if you can and see if it happens. if original screen lcd was damaged/non functional then it may be safe to say it’s a board issue, though you could probably try 1 more screen just to be safe.
keep in mind if you insert an isesamo into the usual spot on the iphone 7/8 series, bottom left of screen, the home button flex cable is right there and is very easy to damage accidentally. I usually go in from bottom right on these particular devices.
test using voice memos. if you can record voice memos with no static sounding then the mics are okay and the fault likely lies with something in the network capabilities. maybe a new antennae but more likely board damage caused by the water
it will likely need an lcd assembly replacement. you can always try this yourself and source the parts, however it’s not something I’d necessarily recommend for a first repair attempt since it’s very easy to break the face id cable on the screen on these ones. if you go this route I’m sure there’s a guide on ifixit for it and I’d definitely follow it closely. otherwise you’ll want to try to find a reputable repair shop that can repair it for you and get a price quote