Should we expect only people from ILAE to participate in the project?

This is ILAE Wikipedia Project and so there would be people from both ILAE and those who are Wikipedians. Each of them would have a distinct role to play. There are a lot of things that need to be done while developing an Wikipedia article. The role of ILAE people come in while ensuring scientific accuracy of the information contained. That is the reason that they would qualify to be subject-matter experts. Let me assure you that Wikipedia has inherent systems that ensure that quality articles are produced. As you proceed, I am certain that you would get a taste of the process and I am certain that you would love it. Wikipedia is about collaboratively contributing. Jump right in and you will find others by your side.

Can we prepare an article within the scope of this project, publish it elsewhere, and then integrate it on Wikipedia?

You definitely have the option of publishing it elsewhere under a compatible license (such as CC-BY) and then integrating to Wikipedia. Unless you use restrictive clauses like NC (Non Commercial) or ND (Non Derivative), articles published as Open Access articles are directly integrable to Wikipedia. We would definitely help you with the process.

After developing an article on Wikipedia is there a scope of publishing it elsewhere?

There are journals like WikiJournal of Medicine or PLOS One which have accepted Wikipedia articles in the past. There is open scope to develop collaborations with further journals in this regard.

Can I develop the project in my native language Wikipedia?

If you are already an Wikipedian working on your respective language wikipedia, the project can indeed be extended to your language. Working in each language will involve a number of people. If you can pull together the folks, guiding it through would not be a problem and you will always have our support. For example, we have eager people on Arabic Wikipedia who would be taking up the project.