The main one is www.spigotmc.org which contains all of the plugins. While builtbybit.com (mcmarket) and polymart.org are only used for CMI

Yes, getting a plugin allows you to use it on the entire bungee network

If the servers are yours then one license is enough.

At this moment no. Plugins should receive similar if not exactly the same amounts of updates on all sites.

Yes, an additional license is not required for this.

Yes, if they’re under your contract and you understand the risk of them ‘stealing’ it from you and putting You in an awkward situation and legal risk of consequences

Currently no, only PayPal payments are accepted at this moment.

CMI should be updated on all sites at the same time

For core review yes, for personal use no. While the plugin is completely not obfuscated it’s still intellectual property. There is nothing to hide so you can always look inside it to make sure about it. Look but don’t touch. 

Yes, you can check the Zrips Github to see the code.

No refunds. Avoid purchasing what you don’t want to have. 

There’s no tryware, shareware, freeware version of the premium plugins. Use reviews or people who own plugins to get an idea of if you actually need it and if it satisfies your needs before purchasing. 

For a single person, no. Discount can only be in case we have a limited-time discount event.

Not planned, but in case the plugin receives more updates and becomes more valuable then it can increase slightly. 

No, getting a plugin gives you access to future updates. As long as the plugin gets updated.

The main focus and compatibility go to the latest Minecraft versions, older versions might experience some issues and some more broken versions might not even have any support depending on the plugin.

Modded servers are not officially supported in general, so don’t expect a flawless experience with servers like that. 

Some plugins are not dependent on the server’s version and don’t require specific additional work to keep it working as intended. That said, the older version you have more likely will not have all of the latest features. While we try to keep backward compatibility with ancient Minecraft versions at some point we will have to drop its support. 

Only Spigot/Paper server engine versions are supported officially. If plugins work on modded servers then they do. But it is not guaranteed to work due to them being completely different in structure and possibly having different base code which can mess up things and prevent plugin from working correct. 
In most cases plugins do work without bigger issues and most features will be up and running, but at same time its common to experience some minor issues which can create errors in console