Seems your issue is that you are turning a variable output to the line output which basically just sends the full signal through it and then you need to adjust the volume on the speaker. Not sure what you can do there aside from maybe talk to the vendor of the speaker and amp for suggestions.
USB ports are useless for this, they are to either play local media off a drive or for flashing new firmware. Some also have USB ports for optional WiFi sticks.
They should have HDMI ports on them, you have the Fire Stick and Fire TV, both of which are HDMI devices. Plug them into the TV and...
Power settings in Windows will let you limit some general CPU speeds. if you turn off speed step also it will slow down the max speed of the processor.. Another option is to open it, clean it up, apply new thermal paste.
Probably not since you said it runs slow. Two cores and 8 GB is not really good for any type of data processing like Excel sheets with lots of data and calculations. 4 core/8 thread is about the minimum I would use. I did testing between Intel i5 and Intel i7 CPUs and even a one or two gen...
Any of the higher end phones within a few generations are perfectly usable. If you can use the dual sim card for a micro SD it will work just like a normal SD card reader. The spec sheet and Samsung support site should have all the info on it.
It seems like a pretty easy thing, but it's not really easy and seems like no development has happened to make it a function. Look up re-use existing tab in chrome online and you will find some things, most from several years ago, but none are very user friendly.
You would need to take it apart and test the power circuitry and battery with a volt meter to make sure the paths are all good. See if someone did a tear down on one of those to guide you.
Component cables on that other disk, chances are you have the cables connected to the wrong spots. Need a bit more details and maybe pictures of how you have things connected. Have you made sure that other player is working in general?
There are batteries available for that laptop on eBay and Amazon, seems pretty easy to replace. Only issue is that since it's an older laptop, even new Samsung batteries may have been sitting for a while un-used which can cause issues for their working time, and the chinese made clones are...
It may be that the coating or surface plastic on the touchpad got worn off or damaged by whatever chemicals were there. Probably won't be able to clean it off without making it worse.
For just a speaker audio cable, sure it should work fine with a good quality cable with shielding and a bit thicker for less resistance. If you mean some other thing, need to add in details of exactly what you are trying to connect.
Looks like there may be a spot for a drive, does the case have a cutout for it on the side? Not very easy to tell from that video how much space it has for an internal disk drive. Very few non-workstation systems include a disk drive these days.