Sure, but if that occurs the state is now no longer an individual and can never leave. States in the US are not individually parties to any agreements with other countries, so the treaty would have no additional members.

The same thing applies if belgium decides to conquer a few more miles of land. You can leave the EU, no state leaves the USA.

If a state wanted to leave the US it could. The US doesn't have an article 50, but if Texas declared independence, the rest of the US could choose to respect that. Of course the US could decide to send in the troops, but the EU could send the troops to the UK. There might be technical legal differences between the two examples, but at this level its more might is right and the winner writes the rules. Example The Ukraine.

Whether a leaving EU party would remain part of a treaty would depend on whether it was individually named. The UK for example is having to negotiate all trade deals anew as current ones are EU only.

No, a state can't leave legally. There was a theoretical framework that would allow it in the Constitution, but that was decided 150 years ago. The EU has an explicit legal avenue for leaving.

Also, no; the EU cannot send troops to the UK. Individual member states could do so, but not the EU. That may change in the future but at the moment it is how it is.

I don't understand the Ukraine reference; in context it seems like you are saying they tried to leave the EU and the EU sent in troops, but that didn't happen. Could you elaborate?

The Ukraine reference was referring to Russia annexing part of it, and the rest of the world doing nothing.

As I said there might be technical legal differences, but practically speaking at this level they can be ignored. If Texas wants independence, it isn't going to be put off by US laws. The US will loudly point to those laws yes, but whether it does anything will be independent of those laws.