I can’t fully grasp what that huge plunger is doing, even with your description and images. I’m 100% confused

This isn't really meant to be a guide of any kind, it's more of a look-and-see/show-and-tell kind of thing. While some teardowns are perfectly fine for disassembly guide, that isn't really the goal.

I'm not too sure what makes this black rectangle any uglier than the rest available for purchase. The clear back is something of a turn-off, personally, but that's hardly a difficult thing to remedy.

The battery didn't seem too bad but the level of complexity in the screen replacement is definitely enough to drop the score to at least a 5. That said i highly doubt ill ever see one of these.

Removing this tape and such to take the LCD off completely is unneccessary when replacing just the glass and digitizer, the digitizer cable is easily removed without further work to the LCD

Try putting the ring around the camera itself rather than adhesing it to the new screen and trying to line it up. Its a simple solution but I admit I certainly didnt think of it right away and had the same trouble as you

Nearly all the guides end like this, you just need to work backwards through the steps using the new hardware.

I don't know if the S4 is the same in this aspect, but not using LOCA also can cause freezes if the screen is 'pressed' too hard on the S3. That said, the only time I've tried to do the LOCA on an S3, the adhesive got EVERYWHERE. Eventually the touchscreen stopped working and I had to buy a new front housing/LCD/digitizer. I assume I used too much glue, but I also felt the LOCA didn't spread properly, as the top corner of the phone received no LOCA and the bottom seemed to get the majority of it despite using the same technique as many guides I saw. I managed to push and spread the glue to the corner but pushing it in that fashion really helped glue up everything. The screen looked much better with the LOCA instead of just the adhesive along the side, though.

TL;DR: LOCA makes things look and work a lot better, but if done wrong it can really screw things up

Step 2.5: Recover iPod from far side of the room due to suction assisted throwing.

thanks, Apple