Question

Is there any way to create an interactive text box for WordPress readers so that they can experiment changing its content, submit it, and see the result of their text processed (by a backend) without leaving the post?

Answer 1

You may use H5P plugin

Create interactive content in WordPress with the H5P plugin

WordPress is best known as a website content management system, but it also a great learn

https://opensource.com/article/20/5/h5p-wordpress

see the Content-types page

Examples & downloads

H5P empowers everyone to create, share and reuse interactive content - all you need is a web browser and a web site that supports H5P.

https://h5p.org/content-types-and-applications

I hope to help.