I fixed up the prereqs for this guide

From my experience they usually do. However if you find it doesn't stick very well afterwards, you can try using some contact cement to hold it in place.

If it helps any, the part number for the LCD screen is CG2-4025. You can also try just searching something like "Canon EOS 650D LCD screen" on eBay to find a replacement screen.

I've edited the guide so it now points out that cable and mentions that it needs to get reconnected. Also I've found removing the top cover of the camera makes reconnecting those cables way easier and in my opinion is worth the extra time to remove the top cover to make reconnecting them easier.

There are some models of Canon Rebel cameras where the two cables which connect the top cover to the DC PCB board are soldered on instead of connected with connectors. This difference must also be found between different variations of the Canon Rebel T3i too.

As far as the screw threads go, most of the screws will be M1.7, but there will also be some places where they use M2.0 screws too. There may also be the occasional place where another thread type is used too, such as the hot shoe. It's definitely recommended to keep track of where exactly every single screw you take off goes to avoid putting any back in the wrong spot.

I believe you are correct. I haven't specifically worked with a T6i either, but with all the other Canon Rebels I've dealt with, those two screws didn't need to be removed to take off the front cover.

From my experience, that plastic ring underneath the main PCB board that the lens mount attaches to is a pretty common thing to break when lenses gets dropped. Personally I'd just recommend replacing the plastic ring instead of fixing it (especially if more than just one screw mount breaks). The occasional times I've had to replace that part on different lenses, I haven't had much trouble finding replacement parts and the price wasn't too crazy. Price and availability can vary greatly though.

I don't have a manual for this particular lens, but typically it is possible. From what I can see from pictures, there doesn't appear to be any exposed screws to unscrew or any holes on the front ring to be able to use a spanner wrench on. It's hard to tell from pictures, but the model name on the front of the lens seems like it might be a name plate which would be held on with a bit of adhesive. If this is the case, you should be able to peal off the front name plate and there will likely be screws underneath. Taking off those screws would allow you to access the front lens element.

Use caution though: often times there aren't holes the screws are in, but rather slots. This allows the the front lens element to be rotated to different positions. If the lens element isn't put back in the exact rotational position as it was prior to removal, the lens will no longer be properly focused.

I'm pretty sure the fiber optical cable is used for the flash. I believe the camera would still work without it connected, but the flash just wouldn't be functional so it still definitely needs to be connected. Also in reply to @octp16dkuy9dbsl, to connect it you just can use a pair of tweezers to push the cable down into the hole in the connector.