Amir,
Bugs are already reported for some of these issues, but the basic question is - why reimplement it all in the first place? Why not use what the browsers offer as much as possible?
there is actually a possibility that a very atypical way to use contenteditable might make a cameo appearance as a thin surface layer to help with input stuff (see VisualEditor/contentEditable for a prototype). The atypical bit mostly stems from the need to avoid the many problems current contenteditable implementations have, and the desire to support a good user experience while editing *wiki* content. In any case, the editor team is currently hard at work on exactly these problems, as is described in https://www.mediawiki.org/wiki/Visual_editor/Phase_2.
https://www.mediawiki.org/wiki/Visual_editor/Bidirectional_text_requirements and Visual editor/Internationalization requirements are helping a lot in this work, thank you for documenting these things!
Gabriel
(who knows very little about the horrors of contenteditable implementations..)