Yes, the screen won't damage other parts of the laptop unless it was liquid damaged, but physical damage will just be localized to the display. You can replace the display assembly with a few screwdrivers and maybe an hour of time. Buying a whole new display assembly can still be expensive though, so you can find just the LCD part and replace that. Replacing the LCD is an advanced repair though, and I wouldn't recommend doing it unless you're experienced. Keep in mind this will void your warrantee if it's still active. Alternatively, you can get a 3rd party repair shop to replace the LCD for you.
The keyboard could be shorting the power rails responsible for turning the laptop on or resetting the SMC (SMC_RESET_L). Replacing the keyboard will most likely solve this. Fortunately, you can replace just the keyboard, it is quite a tedious job though, as there are many small screws that hold the keyboard into the top case. There are quite a few guides for it on the internet :)
I am assuming that you have a 15" Retina MacBook Pro model between late 2013 and mid 2015 - you wrote A1398 in the question but chose a different model to file the question under. You can replace the connector, however, this will require soldering equipment and some experience with microsoldering. You can buy the replacement connector here: https://beetstech.com/product/isight-camera-wtb-connector
Over time, dust and other debris can clog inside the mechanism and prevent the trackpad from working properly. Also, there is a Torx t6 screw positoned under the trackpad button that can be adjusted. Disassemble the trackpad using this guide, clean everything out and reassemble after aligning the trackpad properly: MacBook Pro 13" Unibody Mid 2012 Trackpad Replacement
The part you highlighted is attached to a large frame thing inside the display assembly. You can access and replace this frame, but you will have to remove the glass and the LCD panel first, which is quite a time consuming repair (MacBook Pro 13" Unibody Mid 2010 LCD Replacement). Then, you can take out the screws that hold the frame in place, heat the adhesive and the frame will separate from the back cover. I don't know of any sources that sell just the frame, so you will probably have to salvage it from another LCD assembly. It's most likely the best for you to just buy a whole display assembly and replace the broken one with it. (MacBook Pro 13" Unibody Mid 2012 Display Replacement)
It most likely is a GPU fault. You might be able to TEMPORARILY revive it by heating the GPU (with a heat gun or by putting the board in an oven, sort of like a reflow), but the issue will return since the problem is in the GPU itself, not in the solder balls that attach it to the board. Apple had a recall program for this very issue, but it expired last year. That being said, you might be able to get it fixed for free by taking it to an Apple Store, if the staff there are feeling particularly generous.
Well, you could have either cracked the glass, or the glass and the LCD panel underneath. If you can still see an undistorted image under the cracked glass, then the LCD panel is fine and you will only need to replace the glass. You can replace just the glass with a heat gun and a suction cup: MacBook Pro 13" Unibody Mid 2010 Front Display Glass Replacement
Motion 5, Adobe After Effects and Photoshop are very processor-intensive applications that also use a lot of RAM. Opening one of these apps will likely use up a big portion of your memory, let alone opening two. The lag that you're experiencing is probably the laptop having to reallocate its memory. If you're going to do heavy video editing and motion graphics on a MacBook Air, then it's best for you to keep only one of these applications open at a time.