The quickest and easiest possible solution would be to force shut all running applications and then perform a soft reset. To perform a soft reset you hold the sleep button and homekey simultaneously for 7 secs or till the screen goes black. Then boot the device and see if the issue is resolved.

There are a couple possible causes for this. First test the mic by opening voice notes and making a new recording, if the sound line moves and it pics up your voice clearly than issue lays with a different mic. There is a ambient noise sensor(diff mic) located in the earpiece section that can fail and cause this issue. If it does not pick up your voice and the holes for the mic are unobstructed, the charge port may need to be replaced as that is where the mic is located. Also, a quick soft reset wouldn't hurt to try either. Hold the sleep button and home key simultaneously for about 7 secs or till the screen blacks out. Then turn the phone back on and check again. Best of luck

If your device is set to backup your phone to iCloud automatically, you should be to restore it to a new iPhone in an almost exact state of the old phone prior to it breaking. Here is a link from Apple explaining in detail the correct procedures. https://support.apple.com/en-us/HT201269 Best of luck

I do not personally do a lot of work with these kindles, but I do know that a few of the components are hard to keep in a reusable condition. However, after a quick search in ifixit.com I found that there are used button assembly's being sold. With that being said I'm left with the impression that it is infact salvageable and more of a suggested part incase your current one gets damaged. Best of luck on the repair

All of the reset options are located in SETTINGS-GENERAL-RESET(located at the very bottom of the list). Then choose the reset option that best suites what you are looking to reset. Best of luck

Replacing the logic board is possible with iPhones. If the device is truly an iPhone 5s, the software is already installed as part of the iOS. However, to have the Touch ID feature functional you will need the original homekey that was installed with that logic board. There is no way to get that feature to work with a different home key. Best of luck

Sounds like there is a digitizer issue happening. I would check for any hairline cracks that are under the glass. If there are any, the screen assembly would need to be replaced. Second thing to check which is much more complicated and could cause you to need a new display regardless if done wrong would be to remove the display and reseat the display cable on the logic board. Best of luck

Faulty batteries can also cause issues such as boot loops. Meaning the device will start to power up for a few seconds, shut down, and then will continue to do that. If there are any stores that sell battery's near you, I would see if you could get a new battery put in to try and boot the phone. I've seen this issue daily aswell at my place of work and 7 out of 10 times it solves it.

The other few things to check would be the 2 connectors that control the display and the backlight. Those connectors can bend quite easy, verify that they are flat before placing them on the logic board and work from one side to the other to help ensure correct seating. You mention the screen flickered or lit up? Apple can be funny with diagnostic codes. Sometimes the code is displayed via a certain color millisecond splash screen "flicker". Also, the proximity sensor and ambient light sensor can be damaged easily in those repairs and may keep the device from booting to protect itself. Occasionally by disconnecting just the flex cable from the logic board that contains those sensors and the front facing camera, will allow it to boot fully and confirm the issue. Best of luck

How old is the phone and the battery? Had there been any recent possible damage to the device?