The issue may lie in either the inverter or the backlight bulb working intermittently, or some cables are loose. There is a repair guide in the device page for the screen. You can also use the guide to check the inverter and cables while taking the screen out. Toshiba Satellite C55D-B5206 Screen Replacement
The video in this guide shows how to replace the LCD screen, but around 2:50 the power button is shown. Furthermore, at around 3:30 the motherboard is removed and you can see the power button separate with it. You may have to replace the motherboard, or if the cable is detachable then replace the power button. Alcatel One Touch Idol 3 LCD Display Replacement
The teardown video demonstrates how to replace the touch screen and LCD, but at about 2:53 in the teardown video, the power button is featured. From what I see in the video, it is attached to the motherboard by a ribbon cable when the motherboard is removed at around 3:35. You may have to replace the motherboard or remove the ribbon cable to replace it. Alcatel One Touch Idol 3 LCD Display Replacement
The fact that the battery is being drained after 8 hours without use indicates a parasitic draw. Replacing the battery likely will do nothing. Something is completing the circuit back to the battery, indicating a short somewhere. A short may also be preventing the flash function from working. Below is a teardown guide for the phone. I would inspect all components to see if there is anything visibly wrong with the boards and components. iPhone 6s拆解
The error message "monitor out of range" means that the monitor displaying the message cannot display the image due to something like a higher screen resolution or faster refresh rate. Does the image on the second monitor only take up half of the screen, or something similar? The GPU may be having difficulties with a dual-monitor setup.
It sounds like the DS is crashing upon attempting to read the game card. If another game card works consistently then it may be isolated to the cartridge. If it keeps crashing with any cartridge then there may probably be internal damage. Below is a teardown for the 3DS XL, and a guide to replace the motherboard. It looks like the card reader is attached to the motherboard so if there is a problem with it you may have to replace the whole unit. 任天堂 3DS XL 2015 拆解 任天堂 3DS XL 主板更换
Putting the phone in rice probably didn't help. Some components may be shorted internally, so you should avoid using the device or charging it to prevent internal damage. From your description it sounds like there may be issues with the display, as well as more extensive damage. Below is a teardown of the phone, and also some guides to help with liquid damage to phones. iPhone 6s Plus Teardown 修复iPhone 的进水故障 Electronics Water Damage