Hello,
I Have read through your requirement can help you to done as you requested.
Will suggest you to build this function as new plugins that will not been override and can install at many WordPress setup you need.
For more details ping me we can discuss it.
About MySelf:
My self as a web developer having 12+ years of experience In development and web services, Specialization with CMS like WordPress, Megento, Joomla,Neto, Shopify, BigCommerce and Frameworks like CI,Zend,cakePHP and laravel also i'm good with UI/UX using HTML,CSS and jQuery.I can handle this work and will support when ever you needed.
I have been doing WP them, Plugins development from Scratch, adding hook and filters to WooCommerce on custom them and functions.
I'm assure you the quality work and delivery will be done with in estimated price and time. Ping me for more details to proceed.
Thanks & Regards,