For example, does it make sense to buy a non-copyrighted mod and a copyrighted mod? 
How do copyright versions stack?

What is the main difference essentially?

Copyright is shown differently depending on the mod, for example in the Shoutbox is shown at the top right of the shoutbox while in the Likes mod is shown right next the SMF copyright.

So if you install like 5 mods with copyrights, you have links all over the place like a spammy site?

They are shown at the bottom of the site or in places that don't interfere with the functionality of the mod nor the site. If you would like to remove them, a bundle price can be arranged for the copyright removal of all of them.

I was interested in this 5:

- Menu Editor
- Alerts Pro
- Mentions Pro
- SEO
- WYSIWIG Editor

But I have some questions:
1.- Do Mentions Pro integrate with Alerts Pro?
2.- Do WYSIWIG Editor integrate with the Auto-save functionality of Drafts Mod?

1. Yes.

2. No, since that isn't a SMF feature.

So you can create a non-copyrighted bundle for the first 4?

Sure!