Probably a bit controversial (for structured data fans) but think is very handy in some cases where the rich-text simple doesn’t provide the flexibility and formatting options editors need.
The Contentful App SDK is very easy to deal with, however, would be good to have the possibility to have instance parameters at the field level and not only at the environment level.
Yes, would be possible, but keep in mind would not be structured data like you have with the rich editor, on the WYSIWYG editor works well for images because they are just images, a custom entry can be many things, an approach would be to have just a placeholder element with a data attribute containing the contentful if of the entry as an attribute, so when parsing the resulting HTML that would be required to parse and fetch the right data, any way, is not a clean approach.