Is it possible to use public:// for inline images (in text) to protect against changes in config?
Your issue is (currently) covered in the Drupal issue queue here against Core-8.2.x. This was committed to 8.2.x 4 months ago and I would imagine is in core - reading the issue should let you know which parts have been resolved.

I haven't found any non-core workarounds for this issue at present, the basic concept (from D6/D7 is an input filter to translate public:// URIs into canonical URIs when the content is rendered).

Issue Link: https://www.drupal.org/node/2666382

EDIT

It appears that only part of this change has been finalised, there is another issue suggesting that final resolution for this problem will occur with 8.3.x 

Issue Link: https://www.drupal.org/node/2755223

It's possible (based off your description) that things are working as intended i.e. public:// values should be replaced with their canonical counterpart during the rendering. If this isn't the case I'd suggest following up with the second issue link above.