the storage unit is a chip on what is know as the logic board, mother board etc. extraction for use other wise is usually not an option, also the cameras are also connected to the same board and are used through the board. You have some other project you say - maybe you have software that will assist with making these work? otherwise it does not seem practical.
I am a big proponent of extending electronics usually - however a life safety device that protects you and your family should not be altered but should be replaced per manufactures specs - obviously you are concerned about the expired alarm - just changing the sensor may not help -I understand your decorum issue -
you are looking for a screen replacement? There are repair shops that provide them or if you are handy parts available- check parts and labor (you time also) versus the price of a replacement from Swappa or another reputable place.
try replacing the front facing camera flex. This has what is listed in the comments above - usually cost is 3-5 bucks depending where you buy. Also before you ever do any work on a phone - always remember to disconnect the battery.
when replacing the battery -ensure he did not damage the battery connection or the charging port connection. The screen being black may be just a bad screen or also a damaged connection. - re - walk your steps for changing the battery. Also very common - by the battery connection are small components that are easily knocked off the board when changing the battery. Data recovery is not far from reach - however if we can narrow down your problem - maybe we can save sending it out. Data recovery varies in price and there are a couple of reputable shops.
The LCD appears to have fine damage--(can't see it) separation of the glass LCD when they come usually as a unit can cause this. Have seen this before, there should not be any board damage unless noted during original drop to cause break to begin with-otherwise double check for the slightest amount of debris in the connection.
believing you disconnected the battery and did not cause any damage to the connections -try another screen. also if your old screen worked but was cracked reconnect it to ensure the connections work and no damage to board