I fix pro audio gear for a living, tinker with just about anything else for fun.
I am also an experienced photographer, and quite familiar with many types of current and vintage photographic gear.
To whom it may concern, I'm based in Italy, and fluent in English, Italian and Dutch.
The only reliable way to get those large philips head screws out is to use an impact screwdriver.
If you don't have one, you can use a normal, good quality philips screwdriver of the right size, and a hammer. Place the tip of the screwdriver firmly in the screwhead, grab the handle firmly with one hand, and hit the end of the handle with the hammer while you torque the handle counterclockwise. Apply torque only the moment the hammer hits the handle. Takes a bit of practice but generally works well.
Fixed that, the orange marker was a bit off, it was on an alignment pin instead of the actual screw which was just a bit more to the right.
This guide gets an upvote.
It's the first camera disassembly guide I've seen so far on ifixit that includes the single most important precaution: the camera has a body cap fitted to avoid dust, dirt or other objects getting into the mirror box. Bravo!
The EOS 30 is a film camera and therefore does not have an A/V Port, since it does neither Audio nor Video. The socket near the bottom on the grip side is for a wired remote control.
That's a bit of a bugger. The piece that's broken off is most likely the little "hook" on the "door" (the proper name of this "door" would be "back").
Since this little hook is integral with the back, you'd have to replace the entire back.
If you still have the piece that has broken off, it may be possible to glue it back in place, but you'd have to reinforce it with e.g. a small piece of aluminium sheet cut from a soft drink can or something like that. Use cyanoacrylate glue (otherwise known as "superglue") and make sure the pieces are clean and fit perfectly before applying the glue. You need only a tiny drop.
Sorry, but this troubleshooting guide gets an F.
First:
You NEVER, EVER poke at the shutter or the mirror of a (D)SLR, much less force the shutter open with any type of tool. Chances are you'd cause permanent damage, not only to the mirror and/or shutter but to the image sensor (which sits a tiny fraction of an inch behind the shutter) as well. Such damage would almost certainly classify the camera as BER, which is shorthand for Beyond Economical Repair. In other words, it will turn the camera into an expensive paperweight.
Second:
The most common cause of severely under- or overexposed images is simply operator error.
Many owners of entry-level DSLRs like the T5/1200D simply cannot or will not deal with stuff like aperture, shutter speed and ISO. They shoot full auto and that's it. If, for some reason, the camera's mode dial gets set to M, AV, or TV, panic ensues - sometimes to the point that they even fail to recognize the simple solution: set the mode dial back to the green full-auto icon.