> So the only change needed is that error messages can only be cleared
> from the echo area by the user doing some command. Otherwise they
> are collected and shown.

I see now what you mean.  This can be easily implemented with the
following patch.  So you can set `clear-message-function' to a function
that returns a non-nil, and the echo area won't be cleared.

Such predicate function could contain a complex logic, but for testing
you could use just:

  (setq clear-message-function (lambda () t))