With minor work and the removal of Special:MobileEditor this seems possible.
I started a proof of concept patch to demonstrate: https://gerrit.wikimedia.org/r/267199
- Add editor ResourceLoader module skins.minerva.fallbackeditor to pages with action=edit
- Style anon warning box with warningbox class (might require change in core)
- lead edit icon should be visible https://gerrit.wikimedia.org/r/347126
- Reduce size of the license text and warning messages making more space for the editing form https://gerrit.wikimedia.org/r/347127
- Decrease size of text area so for logged in users without an anon warning box the buttons below it are visible on a standard mobile screen.
- Update MobileFormatter to output action=edit in the URLs of edit links
- skins.minerva.editor should hijack edit links
- Edit icon should be styled to the right without non-js as it is in JS preferably with icon, but be careful with how many icons are pulled down
- cancelLink, mw-editButtons-pipe-separator and editHelp should be hidden in mobile view - they look cramped.
- Discussion link should not show on editor
- Page actions - edit and watch should not show on editor
- Verify non-JS editing works on beta cluster
- Test run on one production wiki
- Remove Special:MobileEditor