It is possible to set ID of cells using the advanced tab of cell properties but as you say headers are not editable anywhere (Alteast I am also unable to find it).

The options I think are ofc. using the old editor instead of the new (not a good option though).

 Otherwise you need to create a new plugin that has the ability to update these attributes on the tables. Or edit or copy the current cell property plugin and edit so that this works as expected. It migt be dangerous to edit the table plugin since this is native in TinyMCE and might be changed by Tiny or by EPi in the future. also some parts are deeply integrated in tiny i think. You can also achive the funcionality by editing the "Insert/Edit Attribute" plugin for tinymce and add headers there.

This might not answer your question completely, but as stated here:

"While the scope attribute is well supported by most screen readers available nowadays; the support was somewhat limited in the older screen readers (< JAWS 6,though it was technically supported since version 4.5 it was very patchy). So using the combination of headers and id may be better if you wish to support the older screen readers, but in general using scope attribute is also fine for marking up tables."

Please let us know if you found a better solution or customized the Insert/Edit Attribute plugin, if you are willing to share it, of course.

Best regards,

Marija