Dear Sir,
Hope you are very well.
My name is Rohit and I am a Full stack php developer working as a Full time freelancer. I have 5+ year experience of web development. I have worked in HTML, CSS, Bootstrap, jQuery, ReactJs, Laravel, codeigniter, Magento, Mysql, Git etc.
I can assist you to complete this POC. We can use latest Facebook library v3.2 for this purpose. As far as I know, user can only post on the pages where he is the owner. User can not post on other liked page. Although, I will check it again and will share my findings if it is possible.
Thanks and looking ahead to have a positive response from your side.