Im not really sure what your use case would be here? If you dont want it in a platform dont use it, its not as if you can accidentally Use it as it relies on you holding a button first. Its like that for a reason and can be easily ignored if you dont want to use it.
You will need to adjust it on a per emulator basis. There is no way a emulator should take more than 30 seconds to load a game unless its cemu or rpcsx3 that are caching shaders first.
Most emulators are portable, some may store some data in your %appdata% folder like Dolphin, but most are just zips and can be stored anywhere when unzipped.
Just tested Need for Speed Most Wanted and got into a race just fine, i have also played both of the God Of Wars before with no issues. You using the retroarch core or standalone PPSSPP?
The startup screens do have code to stop multiple launching in them, thats why i suggested that they will do what you want. They will show a image/images of the game being loaded AND should stop multiple games being loaded.
Those are very low specs you have there to be honest a very old dual core at only 2.0Mhz will be slow for most things including likely actual emulators as well.
All of the things you mention can be done by using the "Startup" screens that were added a few versions back, but you need to have the emulators in fullscreen. I dont think very many people run the games windowed to be honest with you so that may be the issue you have, just run them fullscreen.
Go to Tools/Manage Emulators/MAME and remove the command-line there, you will see that it is what the error is referring to. It should work as normal after removing it.
I did say it may be needed, what i didnt do however is tell you to rush out and buy it immediately. I would refund it for the moment, and re-buy it when @Zombeaver better explains.
Yep, its a known issue with steam link, it knows that you launched launchbox/bigbox, it doesnt however know that that program then launched another (the emulator). You need to look into a paid steam app called "Virtual Here".