1. They mention a 40 to 4 fps drop. I find that very surprising. The only user who included his log.txt has a not very powerful machine that does not follow Laminar Research hardware requirements. So I wouldn't worry too much about that. That being said, one thing is sure, your scene contains a loooot of objects. When reaching that level of details, it can be interesting to have some objects being displayed only for certain levels of graphic settings. This can be done in WED.
2. Your scenery, as published, is only for x-plane 11. So trying to run it with x-plane 10.51 will give errors. To make it 10.51 compatible, it has to be developed inside X-plane 10.51 as I previously said.
The other errors come from world-models library, as I also mentioned before, which brings us to point 3.
3. Yes, and I would strongly advise to do so whith world-models and the other libraries you don't use a lot. X-publish will help you target at them.
To do so, place the library in your custom scenery, remove its library.txt file and modify the path in WED.
X-publish will then help you automatically remove the unused objects and compile a package to be shared.
If you choose to do this, it's very important to have the permission from the library authors, to redistribute them.