If you are gentle, most of the keys are fairly simple, but you want to ensure to pop them off of the wire spring first, then they tilt out. Gentle lifting with a flexible metal bit, or being extremely careful with a standard pocket knife or thin screwdriver may allow you to lift the key up enough to see which side snaps into the wire, and which slides into pockets. Alternatively, you might find an entire replacement keyboard on Amazon or Impact Computers.

Without opening and viewing the boards, this is a tough one to answer. High pitched noise may be a servo motor that is unhappy, multiple components also make sad noises, just before they let the smoke out. If the board is unresponsive to the power switch, then something has gone wrong somewhere in the logic board. Be careful on the insides of these, as the batteries hold a bunch of energy, and we have all seen the videos of these types of boards having much flame come out when the pixies inside are angered.

Those plugs are for all the indicators (turn signals, high beams etc) as well as the ignition system. All must be plugged in to their counterparts in the dash for those systems to work.

Usually, if it is broken on the main case, you would have to get a replacement from someplace like Impact Computers, and move all the internals to the new case, or use (carefully) a plastic welder, available at Harbor Freight.

Multiple reasons could make a system non-bootable. Ensure all USB ports are empty, as the computer could be attempting to boot from a pen-drive. Alternatively, you can make a linux boot usb drive, to see if your hard drive is even being seen by the BIOS. Without being able to put hands on computer, the reasons for this error are many.