Between a third-party application and an in house proprietary application; there is the world of open source. So you don't have to re-invent the wheel.
So the question is not CMS or something else it is what kind of CMS? I hope it is not a they write html, which they are not good at and you upload that HTML unto the server CMS that would be a Dilbert content management system, (no offense intended).
Workflow from Gui Editor to published content?
As Stephen pointed out in the comment. Anything that runs on the site to publish content is called a Content Management System. If you build one from scratch it will still be called a proprietary CMS by everybody else.
If what we are calling a CMS is Wordpress or Drupal, both of them are a bit much for somebody with only design experience. I'm thinking he wants the CKEditor without the complicated back end.
The back end, (what runs on the server), can be PHP or jsnode. Aloha Editor would be a jsnode backend. I'm not familiar with it. Saving the files on the server would for me be quickest using PHP, and then using PHP to embed the content to the pages. A very vanilla but completely functional open source exists which is easy to use to go rouge.
An example, not a recommendation
My old going rouge CMS has been getsimple. I'm not recommending it but using it as an example. It would take me a lot of hours to build a back end from scratch instead of using what is on the shelf.
In the case of getsimple there is nothing other than php it depends on. The output is saved as XML files. No relative database system build on SQL ... just a flat database design where the HTML is saved in an XML file. So the design team can work in a sandbox either on the same host or their own systems and the XML files are transferred or moved.
The intended workflow, for this example CMS, would be that unpublished content is marked as private, which is the simplest way to allow others to create content without the need to move files, etc. Their mindset is keep it simple; Marking something as private is the simplest sandbox implementation.
This would be the data structure.
<?xml version="1.0" encoding="UTF-8"?>
GUI back end is a minimalist CKEditor but can be expanded or made as complex as needed.
Again review open source solutions
To see what you can use so you don't re-invent the wheel. What does your boss want as an editor and where does he want the product produced by the design team to go?
Also what does the design team want?