You find it difficult to explain because it is still unclear what you are trying to achieve, even for you.
I think you should go over each of the obstacles/problems you face and formulate a clear question and ask again.
The way this is brought up, we could have millions of answer and a lot based on opinions and that is, like you said it, considered off topic here and for good reasons.
It’s fairly easy to check a user role/capability and grant or restrict access to certain features, categories, settings, post type, etc. and etc. based on that.
You are saying that you don’t want authors to modify /legal page, /contact page, etc. This looks like a job for current_user_can( 'manage_options' ) checks (only admins can manage options, not authors) and show, hide, redirect, according to capabilities.
You say search form on page A should return page A results. This looks like a job for a custom query maybe filtering by categories.
So again, I would dissect your project and ask questions related to specific obstacles you face so we could better assist you on those specific issues. And remember that big sites and business are also using WordPress in complex and less complex situations, so a lot is possible when we our end objective is clear.