Why not use ssh certificates at that point?
Where “but not to C” is the reason for existence.
How certificates simplify that part?
(Never used them, but my understanding they are usefull when you want x1,x2,… ssh’ing into y1,y2,…; two uniform sets; if set sizes approach 1, then cert usefullness aproach zero)
Ssh certs weren't used because the system was put in place before they became commonplace.