OctoberCMS for the win, if you think it's too much developper oriented, take a look at static page plugin, or content editor plugin which allow easy edit for clients.
Every people that take time to understand it's concept never go back
From the experience of someone who has built a fair few websites with Grav, and maintains several plugins and themes for it, I'll try to answer points 1-7:
Whilst the Admin-plugin contains just about every setting, you can change what is available for what user by creating groups with specific permissions. Editing a Page in Grav is a very similar experience ...
There is a bit of a learning curve, but Postman is great and you could do the stuff you want for free. I use it every day to test urls and api end points.
You can use variables in the url or in headers. You can have different sets of variables for different environments. You can do it really simply, or create tests to check for ...
I don't know a fully-fledged solution for your case, but Linqpad can help. It's gratis and works on Windows 10.
The following script will convert merge an alpha color with a background color and output the resulting hex code:
// Define input here
var bgcolor = System.Drawing.Color.White;
var alphacolor = System.Drawing.Color.FromArgb((int)(0.55*255), 0, 0,...