Can I use an iPod nano 8g as a flash drive for windows, and still use it to play the music I paste in that drive? I know I can use an iPod nano as a flash drive by connecting the iPod to the computer and selecting the iTunes option "Enable disk use".
(as stated here https://discussions.apple.com/thread/3091809?tstart=0)

My question is, after doing that, if I paste an mp3 song from the windows folder explorer (I mean, without using iTunes).
Will I be able to play that song on the go with the iPod?

My goal is to make an iPod nano behave just like any other mp4 player, play any song I copy from my PC without using iTunes, and also use it to take other files with me.
No.
Apple does not want you to manage your own music.
Apple specifically engineered their products to prevent you from easily dragging and dropping files for quick portable usage.
Reasoning behind this is to encourages more sales from iTunes by forcing users to only use iTunes to manage music.
To quote Apple,

When you use your iPod as a hard disk, you can't see the media iTunes
copies to your iPod in the Finder or My Computer. This is normal.
Using the Finder or My Computer, you can't copy these media files from
your iPod back to iTunes or to any other computer. The synchronization
between your iPod and your computer is one way: from iTunes to iPod.

https://support.apple.com/en-us/HT201567
A warning: Keep your backups handy because once your music is in the iPod it's difficult or impossible to get back