My current RAM is two 4 GB modules (2x4) by default - total of 8 GB I bought the same frequency two 8 GB modules (2x8)...

I dont know how you lost the screws all, but anyway in Toshiba Laptop before they have Type of screw in each hole of the laptop F8 is a common screws of them outside the case, inside screws like mounting the Motherboard are short 3mm to 4mm, it depends you have to be very carefull when choosing the right screws as it may damage the board if you wrongly use long screw, you can buy in Amazon the universal screws just make sure to gauge the length you need before forcing a long screw that will hit your motherboard. I hope this helps you out. Good luck. here is the link https://www.amazon.sa/-/en/Anself-300Pcs-Screws-Universal-Computer/dp/B08BG5NNJW/ref=asc_df_B08BG5...-21&linkCode=df0&hvadid=453186851945&hvpo...-1102838879979&psc=1

Normally Heating devices are protected with what we called “Thermal Fuse” which are connected in Series to the Main Line, If you can locate it it looks like a small fuse, crimped in a wire sometimes and comes in a different color like yellow, white or black. sometimes it is covered by a shrinkable tube if you know it. If you can use a Multimeter you can test the Leads of the Heating Element from end to end there should be a low resistance reading from your meter should appear, if it not reads at all then your heating filament is open. Its a sealed type. please think always Safety do not work on it with Power connected , be sure Unplug the Power and ask for a qualified serviceman if your not sure on doing it. Have a nice day.

The negative High Voltage are not present will results uncooked toner, print out will not come out right, you might miss something , if you have patience try to dismantle it again and carefully check everything and it should be all in proper place., check all connectors, sensors, check also the fuser assembly connectors. have a nice day.

If you tried everything on the settings and it does not help, then its time to restore Factory default on your device. all files will be deleted once you done this, be sure to back up everything. if you dont want to format , try searching 3rd party Keyboard apps on internet Have a nice day.

When it cools down , you can you use for 15-20 minutes , the Processor will send signal to Protection circuit to shutdown when it reach the maximum tolerance of over heating, it means you have a defective cooling system, e.g., Fan is stuck-up or full of dust and can't rotate, try cleaning it or replace with new one, I encountered hundreds of cases like this. just give a try. good luck

Power Socket or plug Issue , Just make it sure that it has no loose connection, Please check also the Proper Voltage from your Main AC Source . 220 VAC or 120VAC Extreme carefully handling Power, make sure safety first, avoid electrical Shock. No Wet Floor and Foot upon plugging and unplugging AC Mains.