Hi Toan! Thank you for sharing your thoughts! I completely agree that the proof-style practice problems do not fit well in the Explanation part. At the same time, they also do not fit well in the Problems part either, because the Problems part focuses on measuring what you understand based on the problems that you successfully solve.
We don't really have a good place for such problems on the main Expii right now, and we're focusing on building problems that can be used to measure more basic understanding. If you'd like to discuss more advanced topics, I'd suggest posting them on Art of Problem Solving for now.
That said, I understand a particular draw of Expii, in that we allow you to organize the problems by specific topic area. If you are looking to do that, I think that the best way forward would be to put them into the Explanation part, but with a clear opening sentence that says this is a proof problem, not an explanation. At the end of the problem, you could even provide a link to a forum page discussing it. We created forum.expii.com to permit direct links from outside, without requiring any login. So, you could for example create a new discussion thread in http://forum.expii.com/category/12/advanced-topics and allow people to discuss it there. Or, you could direct-link to Art of Problem Solving!
Someday, as Expii continues to grow, we'll provide a richer problem area. For now, we're focusing on serving less experienced learners, so that there can be more people able to enjoy the advanced problems!