If you replace the front camera assembly with that part your Face ID will not work. Apple does sell the TrueDepth camera assembly o the self service repair site but it is significantly more expensive. Buying the new part from Apple and using their service to pair it with your device is the only way to replace the TrueDepth assembly and retain face ID.

What you're describing typically is what happens when the battery is not plugged in. I would check your battery connection first and look for any damage to the connector or the board in that area.

They are basically the same. Unless you are experienced at replacing screens I would save yourself a lot of trouble and be sure to order the new screen with frame. This will make the repair a lot easier and is easily worth the slightly higher price.

You might have ordered a digitizer with a home button for an iPad Air pre-installed. These are common because the Air does not have touch ID. You have to order a home button specifically for an iPad 5.

The first one is a sim eject pin but used to be called the sim eject lever on older models. I have only seen this part included in small parts kits sold on eBay for $7 or so. The second pic is called the wifi flex to charging port transition flex and is available from most parts suppliers.

I would look for a replacement flex cable elsewhere. Every one I have purchased never required any soldering. My cables I have in stock now have the switch attached to the cable already.

The plastic frame around the screen is where the 2 threaded holes are for the bottom screws. if the pieces have been broken off the only way to properly fix it is to either separate the frame from the glass and replace that part (this is not an easy option). Your other choice is to replace the screen with a new assembly. This is an easy option and should not be too costly for an iphone 6 since there are lots of options for screens. You can even buy the parts from ifixit if your concerned about buying from a reputable seller but you can also buy from other sources just do some research on the seller and the quality of their parts. The last and probably the cheapest option is to simply use some adhesive like tesa tape or similar to attach the screen. This is not a very secure solution but if you just want a cheap fix it will work. There are a lot of different adhesives available but the phone would still be able to separate the screen easier than it would with a new screen. You would also need to buy a pair...

The fingerprint sensor is replaceable and readily available from many sources. Most phone parts suppliers have them in stock. You can also find them on ebay or amazon but make sure you only buy from a reputable seller on these platforms. Replacing the part is fairly easy especially since you already have the phone open.

Your broken screen likely caused a short. The most likely is a short on vcc_main. There are a lot of components that can short that receive power from vcc_main. You will likely need to find a repair shop that does micro-soldering to fix your phone. Given your description I would say its pretty unlikely you have done irreversible damage to your phone which is the good news. If you have a shorted component you will have to clear this short before your phone will power on. You also should not try to charge the phone until you resolve the issue so you do not cause any further damage to the phone.

You most likely have a short somewhere on the board. You need to hook it to a bench power supply and see if it is drawing any current without being powered on. If it is then your primary short is going to be on a main power line (likely vcc_main). A favorite capacitor to blow with water damage is the one on the back of the board near the wifi chip. It is very close to the sim slot which is why it blows typically with water damage. You will need microsoldering equipment to remove and replace this component. You can remove it and see if your short is cleared. Your phone will operate without this capacitor but if you plan to keep using it I would recommend you simply remove and replace the component.