1. Yes. I use the default templates by ParasiteX on MTS2, but in the past I have used other people's defaults as base.
2. If you extract the S1 - S4 skintones while you have a default replacement in place, you will get the textures for the default skintone and not the Maxis skintone textures. If you want to get the Maxis skintone textures you need to remove the default replacement. I don't know why you'd want to extract the default textures (Maxis or replacement) because you don't need them to make a default replacement. They are what you are trying to replace not use.
3. The game calls on the information that is inside the file not on the filename. I have used defaults in the past that did not have the word "default" in the filename and they worked just fine. Including the word "default" and the skintone it refers to, either "light" or "S1", etc, is more of a naming convention the community has adopted. It helps to identify what is inside the file and is especially useful when you want to remove the replacement later.
To make a default replacement you need a template (either you make your own or use someone else's) and a custom skintone you want to use. After you have those, you basically need to follow the tutorial you have been linked to.
Parasite-X templatesETA: Be careful what replacement base you use. Some people use what is called a "dependent default" which only replaces the face textures and calls on the custom file for body textures. This is why I gave you the link to Parasite-X's templates. People don't always say they are using the dependent default method and just say they are giving you a "default add-on". I think that it would probably be best if you just use the templates I linked you to rather than accidentally end up with a dependent default template. If you are wanting to make supernatural replacements, feel free to grab any of the defaults I have posted on this site. I use the texture replacement method like in the tutorial, so you won't get a dependent replacement from any of my files. Also, make sure to use the Compressorizer on the final file. This will greatly reduce the size of the file. Depending on which settings you use to import the textures, you could end up with a 30MB replacement file. The Compressorizer will squish it to below 10MB.